Implement the 006-pre-commit-gate feature that enforces a pre-commit quality gate using Lefthook to run pnpm check before every commit

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/006-pre-commit-gate/quickstart.md

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+# Quickstart: Pre-Commit Gate
+
+## What This Feature Does
+
+Adds a Git pre-commit hook (managed by Lefthook) that runs `pnpm check` before every commit. If any check fails, the commit is blocked.
+
+## Files to Create/Modify
+
+| File | Action | Purpose |
+|------|--------|---------|
+| `lefthook.yml` | Create | Lefthook configuration with pre-commit hook |
+| `package.json` | Modify | Add `lefthook` devDependency + `prepare` script |
+
+## Setup After Implementation
+
+After the feature is implemented, hooks activate automatically:
+
+```bash
+pnpm install    # installs lefthook + runs `prepare` which calls `lefthook install`
+```
+
+## How It Works
+
+1. Developer runs `git commit`
+2. Lefthook intercepts via the Git pre-commit hook
+3. Lefthook runs `pnpm check` (format + lint + typecheck + test)
+4. If `pnpm check` exits 0 → commit proceeds
+5. If `pnpm check` exits non-zero → commit is blocked, output shown
+
+## Bypass
+
+```bash
+git commit --no-verify    # skips the pre-commit hook
+```