feat: add marketplace skill discovery to planning flows

[malikrohail]

Summary

Closes #419.

Adds a marketplace-aware recommendation layer for planning/design flows plus a manual /find-skills command for parity with Agent Skills / skills.sh search.

What changed

• Added bin/gstack-marketplace-search.ts
  • queries the marketplace via skills search <query> --json
  • falls back to npx -y agent-skills-cli search <query> --json
  • strips CLI prelude noise, normalizes results, ranks matches, and caches responses under ~/.gstack/cache/marketplace-search/
• Added a new manual skill:
  • find-skills/SKILL.md.tmpl
  • find-skills/SKILL.md
• Added a shared generator resolver in scripts/gen-skill-docs.ts
  • injects an External Skill Marketplace Check into:
    • /office-hours
    • /plan-ceo-review
    • /plan-eng-review
    • /plan-design-review
    • /design-consultation
    • /autoplan
• Updated docs / registration surfaces:
  • root SKILL.md.tmpl
  • README.md
  • AGENTS.md
  • CLAUDE.md
  • docs/skills.md
  • scripts/skill-check.ts
  • test/skill-validation.test.ts

Behavior

When PROACTIVE=true, the planning/design skills can now:

1. take the original user request or current plan/design summary
2. extract concrete technology keywords
3. query the external marketplace
4. surface the top matching external skills with install commands

This is recommendation-only in v1:

• no auto-install
• no auto-run of third-party skills
• silent fallback when the marketplace CLI is unavailable

Manual parity path

Users can now explicitly run /find-skills <query> to search the marketplace on demand.

Related context

Related but distinct from #113:

• Make individual skills available via skills.sh #113 is about publishing gstack’s own skills to skills.sh
• this PR is about gstack consuming the marketplace to recommend third-party skills

Verification

• gh issue create --repo garrytan/gstack ... created Feature: marketplace-aware external skill recommendations in planning/design flows #419
• bun run scripts/gen-skill-docs.ts
• bun run scripts/gen-skill-docs.ts --host codex
• bun test test/marketplace-search.test.ts
• bun test test/skill-validation.test.ts -t "find-skills skill structure|marketplace skill discovery integration"
• bun run skill:check
• bun run scripts/gen-skill-docs.ts --dry-run
• bun run scripts/gen-skill-docs.ts --host codex --dry-run
• bun test

Notes

This PR intentionally stops short of auto-installing external skills. The recommendation layer is meant to improve planning quality without adding hidden side effects or third-party execution risk.