Add pluginpool (10 productivity helpers, stdlib only)

[mturac]

Adds pluginpool under Development & Code Tools.

Ten focused developer productivity helpers — each one a deterministic Python script that does exactly one thing and emits structured output. Use as plain CLIs from any Codex session, or install as Claude Code slash commands.

• commit-narrator — semantic commit message from git diff --cached, with the why
• pr-storyteller — PR title + body + test plan from commits and diff vs base
• test-gap — lines in diff lacking test coverage (Cobertura / lcov / coverage.json)
• deps-doctor — multi-ecosystem dependency audit (npm, pip, cargo, go)
• env-lint — .env vs .env.example key parity (never prints values)
• secret-guard — pre-commit secret scanner (pattern + entropy, redacted output)
• standup-gen — daily standup notes from git activity across repos
• todo-harvest — TODO/FIXME/HACK scan with git blame author + age
• flaky-detector — run a test command N times, report per-test flakiness %
• changelog-forge — conventional commits → CHANGELOG section + semver bump

89 hermetic tests across the suite, Python stdlib only at runtime, MIT.