feat(skills): add model-invocable SkillTool

[yqmai]

Summary

Adds model-invocable skills through a new SkillTool execution path.

• Adds explicit skill metadata for model invocation eligibility
• Runs model-invocable skills in isolated child Engines through SkillRunner
• Adds SkillTool with frozen authorization schema and structured results
• Preserves manual slash skill behavior
• Adds worker skill authorization for Coordinator workers
• Keeps Coordinator, Explore, and Plan Mode boundaries enforced

Safety / Permissions

• Skills are not model-invocable by default
• A skill must explicitly set model-invocable: true and declare allowed-tools
• SkillTool does not bypass existing tool permissions or sandbox rules
• /commit and /simplify remain manual-only
• Plan Mode and Coordinator main agents do not expose SkillTool
• Worker skill grants are fixed at worker creation

Tests

• pytest tests -q
• Diff whitespace check passed