Dev: Add Husky + lint-staged pre-commit hook

[evans-sam]

Problem

Nothing runs locally before a commit. Formatting and lint failures are only discovered in CI, causing avoidable round-trips.

Why it matters

Fast feedback at commit time > slow feedback at PR time. Also standardizes the dev experience across contributors.

Proposed approach

1. Install husky and lint-staged as devDependencies.
2. Add .husky/pre-commit running npx lint-staged.
3. Configure lint-staged in package.json:

"lint-staged": {
  "*.ts": ["eslint --fix", "prettier --write"],
  "*.{json,md,yml}": ["prettier --write"]
}

4. Add postinstall script: husky install.
5. Document in CONTRIBUTING.md.

A superpowers:setup-pre-commit skill exists that does most of this — consider invoking it.

Acceptance criteria

• Committing a badly-formatted file triggers the hook.
• CONTRIBUTING.md mentions the hook and how to bypass (--no-verify) with caution.

[evans-sam]

Coordination note: see #31 (Biome migration).

If Biome lands first, the lint-staged config in this issue should be:

"lint-staged": {
  "*.{ts,js,json,jsonc}": ["biome check --write"]
}

…instead of separate eslint --fix + prettier --write. If this issue lands first, the lint-staged config will need a follow-up swap when Biome migration completes.

Note that Biome doesn't format .md or .yml, so the *.{json,md,yml} glob in this issue's current proposed config needs to be split if we keep formatting docs.

[evans-sam]

Update: see #32 — Lefthook supersedes the Husky + lint-staged tooling proposed in this issue. This issue still owns the spec for what runs in the pre-commit hook (format, lint); #32 owns the framework that runs it.

Net effect on this issue's "Proposed approach":

• Steps 1, 2, 4 (install Husky, add .husky/pre-commit, add postinstall) → handled by Dev: Use Lefthook for git hooks (instead of Husky) #32.
• Step 3 (the lint-staged config) → folded into lefthook.yml per Dev: Use Lefthook for git hooks (instead of Husky) #32.
• Step 5 (CONTRIBUTING.md docs) → still owned here, but referencing lefthook instead of husky.