chore: add dependency-free pre-commit hooks (SPDX + cookbook drift)

[w1ne]

What

Adds a lightweight, dependency-free git pre-commit hook that catches — locally, before push — the two "forgot to regenerate the derived artifact" failures that have repeatedly wasted CI on PRs:

• (a) Missing SPDX headers on new/edited TS source under src/.
• (b) Stale cookbook artifacts (src/agent/skills/kernelcad-authoring/SKILL.md) when their cookbook source (src/agent/cookbook/**) is part of the commit.

How it works

• No dependencies / no husky. Wired via git's native core.hooksPath, pointed at the committed .githooks/ dir by the prepare npm script (git config core.hooksPath .githooks || true; npm run build:studio). npm ci / npm install sets it up automatically; the || true keeps CI/non-git checkouts from failing, and it composes with the existing build:studio prepare step.
• Fast & staged-scoped. It only inspects git diff --cached --name-only --diff-filter=ACM:
  • SPDX: runs node scripts/addSpdxHeaders.mjs only when staged src/**/*.ts(x) exist, then re-stages only the staged files it actually touched (never sweeps unrelated working-tree changes). Idempotent — quiet when nothing changes.
  • Cookbook: runs npm run cookbook:build only when a src/agent/cookbook/** file is staged, then re-stages the regenerated SKILL.md. It never runs on commits that don't touch cookbook source.
• Skippable. git commit --no-verify bypasses it via git's standard mechanism.
• Robust on partial checkouts. If node or the scripts are missing, it warns and exits 0 rather than blocking commits.

Behavior choice (auto-fix vs fail-fast)

The hook auto-fixes and re-stages rather than failing, because both fixers are deterministic regenerators (insert canonical header / regenerate a <!-- COOKBOOK --> block) — fixing in place is faster and less surprising than aborting the commit and asking the dev to re-run a command by hand. Notices are printed to stderr so the dev sees what was added.

Verification

• bash -n .githooks/pre-commit — syntax OK.
• Functional test: created a header-less src/studio/__hooktest__.ts, staged it, ran the hook directly → it inserted the two-line SPDX header and re-staged the file (working tree matched index afterward). Temp file removed, not committed.
• core.hooksPath resolves to .githooks after the prepare wiring runs.