fix: add missing skill frontmatter

[nyldn]

Summary

Thanks for publishing these Claude Code plugin examples. While validating the plugins locally, I found that a broader set of skill entrypoints were missing YAML frontmatter, not just the previously reported thesis-tracker case.

This PR:

• adds YAML frontmatter with name and description to the affected vertical skill sources
• syncs the bundled agent-plugin skill copies from those vertical sources
• extends scripts/check.py so future plugin skill entrypoints must include documented skill frontmatter metadata

This intentionally does not touch the existing hooks.json array schema problem or README URL problem, since those already have multiple open issues/PRs.

Related: #18 and #36 cover one affected skill; this broadens that fix across the remaining skill files.

Validation

• uv run --with pyyaml python scripts/check.py
• python3 scan of plugins/**/skills/*/SKILL.md confirmed missing_frontmatter=0
• git diff --check
• claude plugin validate plugins/agent-plugins/earnings-reviewer
• claude plugin validate plugins/agent-plugins/market-researcher
• claude plugin validate plugins/agent-plugins/meeting-prep-agent
• claude plugin validate plugins/agent-plugins/pitch-agent
• claude plugin validate plugins/agent-plugins/valuation-reviewer

[nyldn]

Closing as superseded. #243 merged the 41 skill-file fixes, and #327 now carries the remaining regression guard on current main, with focused subprocess tests and no skill-file rewrites.