feat: add skills agents command to sync AGENTS.md

[farnoodma]

Summary

Purpose: emphasize available skills in AGENTS.md, since models are not yet good enough to pick up skills when they truly need them, but they are good at following AGENTS.md instructions; this can be removed once newer models are better trained at using skills.

• add a new skills agents command (alias: skills sync-agents) to sync installed skills into project AGENTS.md
• upsert a managed block in AGENTS.md containing a skills table (name + description)
• support -g, --global to sync global skills instead of project skills
• add command docs in README.md and project AGENTS.md
• add tests for create/update/upsert behavior and CLI help/banner coverage

Testing

• pnpm format:check
• pnpm test
• pnpm build
• node scripts/validate-agents.ts

[vercel]

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