Add pluginpool (10 productivity plugins, marketplace-installable)

[mturac]

Adds mturac/pluginpool — a Claude Code plugin marketplace with ten focused developer productivity plugins.

The 10 plugins:

1. commit-narrator — semantic commit message from staged diff
2. pr-storyteller — PR title + body + test plan from commits
3. test-gap — lines in diff lacking test coverage
4. deps-doctor — multi-ecosystem dependency audit (npm, pip, cargo, go)
5. env-lint — .env vs .env.example key parity (never prints values)
6. secret-guard — pre-commit secret scanner (pattern + entropy)
7. standup-gen — daily standup notes from git activity
8. todo-harvest — TODO/FIXME/HACK scan with git blame
9. flaky-detector — per-test flakiness % from N runs
10. changelog-forge — conventional commits → CHANGELOG + semver

89 hermetic tests across the suite, Python stdlib only at runtime, MIT. Install via marketplace or per-plugin clone.