CI: typecheck / lint / test on push + PR

[axisrow]

Part of #2. PR #1 was merged with zero automated checks — the fork must self-verify.

Что сделать

1. .github/workflows/ci.yml: on push + pull_request → setup Node 20 + pnpm 10 (cache), pnpm install --frozen-lockfile, then pnpm typecheck, pnpm lint, pnpm test.
2. After [FEAT] Layer 2: package the CLI as a Claude Code plugin (marketplace + plugin.json + skills) #3 lands: append claude plugin validate . step.
3. Branch protection is optional (single-maintainer repo), but the check must at least exist and be green on main.

Acceptance

• Green CI run on main
• CI runs on the next PR automatically

[axisrow]

Критерий приёмки хвоста issue («claude plugin validate . в CI, зелёный прогон на PR с этим шагом»): выполнен.

• PR с шагом: feat(ci): add claude plugin validate step #31
• Зелёный CI-run: https://github.com/axisrow/claude-devtools/actions/runs/36131661699
• Job-степ Validate plugin — success; CLI установлен в шаге через npm install -g @anthropic-ai/claude-code (на раннерах не предустановлен).