refactor(skills): move Skills Repository to experimental + CREWAI_EXPERIMENTAL gate

Moves the registry/cache pieces of PR #5867 under crewai.experimental.skills
and the CLI commands under `crewai experimental skill`. The stable local-file
skills feature (loader, parser, validation, models) stays in crewai.skills.

Both entry points now require CREWAI_EXPERIMENTAL=1:
- resolve_registry_ref() calls require_experimental_skills() before resolving
- The `crewai experimental` CLI group raises UsageError when the flag is unset

SkillDownloadStarted/CompletedEvent move out of crewai.events.types.skill_events
into crewai.experimental.skills.events.

* refactor(skills): move 'version' off SkillFrontmatter into metadata

The skill version is now stored as `metadata.version` rather than a
top-level field on `SkillFrontmatter`. A `before` validator lifts any
top-level YAML `version:` into `metadata['version']` so existing SKILL.md
files keep parsing.

lib/cli/tests/experimental/skills/test_main.py

@@ -36,7 +36,7 @@ def skill_command():
             TokenManager().save_tokens(
                 "test-token", (datetime.now() + timedelta(seconds=36000)).timestamp()
             )
-            from crewai_cli.skills.main import SkillCommand
+            from crewai_cli.experimental.skills.main import SkillCommand
             cmd = SkillCommand()
             yield cmd
 
@@ -142,7 +142,7 @@ class TestSkillPublish:
                 mock_resp.status_code = 200
                 mock_resp.json.return_value = {}
                 mock_client.publish_skill.return_value = mock_resp
-                with patch("crewai_cli.skills.main.Settings") as mock_settings_cls:
+                with patch("crewai_cli.experimental.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):
@@ -151,14 +151,14 @@ class TestSkillPublish:
     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."
+                "---\nname: my-skill\ndescription: A test skill.\nmetadata:\n  version: 1.0.0\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:
+            with patch("crewai_cli.experimental.skills.main.Settings") as mock_settings_cls:
                 mock_settings_cls.return_value.org_name = "acme"
                 mock_settings_cls.return_value.enterprise_base_url = None