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/src/crewai_cli/cli.py

@@ -17,6 +17,7 @@ from crewai_cli.crew_chat import run_chat
 from crewai_cli.deploy.main import DeployCommand
 from crewai_cli.enterprise.main import EnterpriseConfigureCommand
 from crewai_cli.evaluate_crew import evaluate_crew
+from crewai_cli.experimental.skills.main import SkillCommand
 from crewai_cli.install_crew import install_crew
 from crewai_cli.kickoff_flow import kickoff_flow
 from crewai_cli.organization.main import OrganizationCommand
@@ -26,7 +27,6 @@ from crewai_cli.replay_from_task import replay_task_command
 from crewai_cli.reset_memories_command import reset_memories_command
 from crewai_cli.run_crew import run_crew
 from crewai_cli.settings.main import SettingsCommand
-from crewai_cli.skills.main import SkillCommand
 from crewai_cli.task_outputs import load_task_outputs
 from crewai_cli.tools.main import ToolCommand
 from crewai_cli.train_crew import train_crew
@@ -544,8 +544,19 @@ def tool_publish(is_public: bool, force: bool) -> None:
 
 
 @crewai.group()
+def experimental() -> None:
+    """Experimental, unstable commands. Subject to change without notice."""
+    import os
+
+    if os.environ.get("CREWAI_EXPERIMENTAL") != "1":
+        raise click.UsageError(
+            "Experimental commands are gated. Set CREWAI_EXPERIMENTAL=1 to enable."
+        )
+
+
+@experimental.group(name="skill")
 def skill() -> None:
-    """Skill Repository related commands."""
+    """Skill Repository related commands (experimental)."""
 
 
 @skill.command(name="create")