feat: add Skills Repository — registry, cache, CLI, and SDK integration

Adds a Skills Repository feature allowing users to publish, install,
and use skills from the CrewAI registry with @org/skill-name refs.

## What's New

### SDK (lib/crewai/)
- SkillFrontmatter: added optional 'version' field (backward compatible)
- SkillCacheManager: manages ~/.crewai/skills/{org}/{name}/ with
  .crewai_meta.json tracking, path-traversal-safe tar extraction
- SkillRegistry: parse @org/skill-name refs, local-first resolution
  (./skills/ > cache > download), interactive prompt on first use,
  CI-mode guard (CREWAI_NONINTERACTIVE/CI env vars)
- Agent.skills and Crew.skills widened to accept str refs (@org/name)
- set_skills() resolves registry refs with org-prefixed dedup keys
- New events: SkillDownloadStartedEvent, SkillDownloadCompletedEvent

### CLI (lib/cli/)
- crewai skill create <name> — context-aware (project vs standalone)
- crewai skill install @org/name — downloads to ./skills/ or cache
- crewai skill publish — ZIP + upload to org registry
- crewai skill list — show installed skills

### PlusAPI (lib/crewai-core/)
- Added SKILLS_RESOURCE, get_skill(), publish_skill(), list_skills()

### Scaffolding
- crew and flow templates now include skills/ directory

### Tests
- 91 SDK skill tests + 15 CLI skill tests, all passing

lib/cli/tests/skills/test_main.py

@@ -0,0 +1,206 @@
+"""Tests for SkillCommand CLI."""
+
+from __future__ import annotations
+
+import io
+import json
+import os
+import tempfile
+import zipfile
+from contextlib import contextmanager
+from datetime import datetime, timedelta
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from crewai_cli.shared.token_manager import TokenManager
+
+
+@contextmanager
+def in_temp_dir():
+    original = os.getcwd()
+    with tempfile.TemporaryDirectory() as td:
+        os.chdir(td)
+        try:
+            yield td
+        finally:
+            os.chdir(original)
+
+
+@pytest.fixture
+def skill_command():
+    with tempfile.TemporaryDirectory() as temp_dir:
+        with patch.object(
+            TokenManager, "_get_secure_storage_path", return_value=Path(temp_dir)
+        ):
+            TokenManager().save_tokens(
+                "test-token", (datetime.now() + timedelta(seconds=36000)).timestamp()
+            )
+            from crewai_cli.skills.main import SkillCommand
+            cmd = SkillCommand()
+            yield cmd
+
+
+# ---------------------------------------------------------------------------
+# create
+# ---------------------------------------------------------------------------
+
+class TestSkillCreate:
+    def test_create_in_project(self, skill_command, tmp_path):
+        with in_temp_dir():
+            # Simulate being inside a project
+            Path("pyproject.toml").write_text("[tool.poetry]\nname = 'test'\n")
+            skill_command.create("my-skill")
+            assert Path("skills/my-skill/SKILL.md").exists()
+            assert Path("skills/my-skill/scripts").is_dir()
+            assert Path("skills/my-skill/references").is_dir()
+            assert Path("skills/my-skill/assets").is_dir()
+
+    def test_create_outside_project(self, skill_command, tmp_path):
+        with in_temp_dir():
+            skill_command.create("standalone-skill", in_project=False)
+            assert Path("standalone-skill/SKILL.md").exists()
+
+    def test_create_adds_name_to_skill_md(self, skill_command):
+        with in_temp_dir():
+            skill_command.create("hello-world", in_project=False)
+            content = Path("hello-world/SKILL.md").read_text()
+            assert "name: hello-world" in content
+            assert "version: 0.1.0" in content
+
+    def test_create_fails_if_dir_exists(self, skill_command):
+        with in_temp_dir():
+            Path("existing-skill").mkdir()
+            with pytest.raises(SystemExit):
+                skill_command.create("existing-skill", in_project=False)
+
+
+# ---------------------------------------------------------------------------
+# install
+# ---------------------------------------------------------------------------
+
+class TestSkillInstall:
+    def _zip_skill(self, name: str) -> bytes:
+        buf = io.BytesIO()
+        with zipfile.ZipFile(buf, "w") as zf:
+            zf.writestr("SKILL.md", f"---\nname: {name}\ndescription: Test.\n---\nInstructions.")
+        return buf.getvalue()
+
+    def test_install_invalid_ref_no_at(self, skill_command):
+        with pytest.raises(SystemExit):
+            skill_command.install("acme/my-skill")
+
+    def test_install_invalid_ref_no_slash(self, skill_command):
+        with pytest.raises(SystemExit):
+            skill_command.install("@acmeskill")
+
+    def test_install_404(self, skill_command):
+        mock_resp = MagicMock()
+        mock_resp.status_code = 404
+        skill_command.plus_api_client.get_skill = MagicMock(return_value=mock_resp)
+
+        with pytest.raises(SystemExit):
+            skill_command.install("@acme/ghost")
+
+    def test_install_in_project(self, skill_command):
+        import base64
+        archive = self._zip_skill("my-skill")
+        encoded = "data:application/zip;base64," + base64.b64encode(archive).decode()
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.json.return_value = {"file": encoded, "version": "1.0.0"}
+        skill_command.plus_api_client.get_skill = MagicMock(return_value=mock_resp)
+
+        with in_temp_dir():
+            Path("pyproject.toml").write_text("[tool]\n")
+            skill_command.install("@acme/my-skill")
+            assert Path("skills/my-skill/SKILL.md").exists()
+
+
+# ---------------------------------------------------------------------------
+# publish
+# ---------------------------------------------------------------------------
+
+class TestSkillPublish:
+    def test_publish_no_skill_md(self, skill_command):
+        with in_temp_dir():
+            with pytest.raises(SystemExit):
+                skill_command.publish(is_public=True, org="acme")
+
+    def test_publish_missing_version(self, skill_command):
+        with in_temp_dir():
+            Path("SKILL.md").write_text(
+                "---\nname: my-skill\ndescription: Test.\n---\nInstructions."
+            )
+            with pytest.raises(SystemExit):
+                skill_command.publish(is_public=True, org="acme")
+
+    def test_publish_missing_name(self, skill_command):
+        with in_temp_dir():
+            Path("SKILL.md").write_text(
+                "---\ndescription: Test.\nversion: 1.0.0\n---\nInstructions."
+            )
+            with pytest.raises(SystemExit):
+                skill_command.publish(is_public=True, org="acme")
+
+    def test_publish_no_org(self, skill_command):
+        with in_temp_dir():
+            Path("SKILL.md").write_text(
+                "---\nname: my-skill\nversion: 1.0.0\ndescription: Test.\n---\nInstructions."
+            )
+            with patch.object(skill_command, "plus_api_client") as mock_client:
+                mock_resp = MagicMock()
+                mock_resp.is_success = True
+                mock_resp.status_code = 200
+                mock_resp.json.return_value = {}
+                mock_client.publish_skill.return_value = mock_resp
+                # No org set → should SystemExit (no org_name in settings)
+                with patch("crewai_cli.skills.main.Settings") as mock_settings_cls:
+                    mock_settings_cls.return_value.org_name = None
+                    mock_settings_cls.return_value.enterprise_base_url = None
+                    with pytest.raises(SystemExit):
+                        skill_command.publish(is_public=True, org=None)
+
+    def test_publish_calls_api(self, skill_command):
+        with in_temp_dir():
+            Path("SKILL.md").write_text(
+                "---\nname: my-skill\nversion: 1.0.0\ndescription: A test skill.\n---\nInstructions."
+            )
+            mock_resp = MagicMock()
+            mock_resp.is_success = True
+            mock_resp.status_code = 200
+            mock_resp.json.return_value = {}
+            skill_command.plus_api_client.publish_skill = MagicMock(return_value=mock_resp)
+            with patch("crewai_cli.skills.main.Settings") as mock_settings_cls:
+                mock_settings_cls.return_value.org_name = "acme"
+                mock_settings_cls.return_value.enterprise_base_url = None
+
+                skill_command.publish(is_public=False, org="acme")
+
+            skill_command.plus_api_client.publish_skill.assert_called_once()
+            call_kwargs = skill_command.plus_api_client.publish_skill.call_args
+            assert call_kwargs.kwargs["name"] == "my-skill"
+            assert call_kwargs.kwargs["version"] == "1.0.0"
+
+
+# ---------------------------------------------------------------------------
+# list_cached
+# ---------------------------------------------------------------------------
+
+class TestSkillListCached:
+    def test_list_cached_empty(self, skill_command, capsys):
+        with in_temp_dir():
+            skill_command.list_cached()
+            # Should not raise
+
+    def test_list_cached_shows_project_skills(self, skill_command, capsys):
+        with in_temp_dir():
+            skill_dir = Path("skills/my-skill")
+            skill_dir.mkdir(parents=True)
+            (skill_dir / "SKILL.md").write_text(
+                "---\nname: my-skill\nversion: 0.5.0\ndescription: A skill.\n---\nBody."
+            )
+            skill_command.list_cached()
+            # Should complete without error