chore(gate): commit pre-push gate (secret -> build -> test, fail-closed)

- scripts/gate/gate.sh: 3-stage gate — 1) gitleaks, 2) build
  (tsc on commit, full next build on push), 3) LLM parse contract
  tests. Fail-closed: setup errors, timeouts, and findings all block.
- scripts/gate/hooks/{pre-commit,pre-push}: exec gate.sh commit|push.
- scripts/install/install-hooks.sh: idempotent installer (verifies repo
  root, bootstraps pinned gitleaks 8.30.1 if absent, wires both hooks).
- scripts/install/bootstrap-gitleaks.sh: pinned per-user install,
  x86_64/arm64, GitHub release download + SHA-less checksum pin.
- .gitleaks.toml: useDefault=true; single allowlist = .env.example
  placeholder lines (secret= and change-me values) by path+regex.
  Real secrets — even inside .env.example — still trip the gate
  (empirically verified: OpenAI/AWS/Slack/GitHub tokens all caught).
- tests/llm-parse.test.ts: pins parse.ts contracts (strict 6-paragraph
  body, headline/scalar/or array rejection, stopword rules, tag
  fallback) — the choke point for LLM output parsing.
- package.json: 'test' script.
- README: 'Commit gate' section (install, stages, verified fail-closed
  modes).

Verified before commit: clean tree PASSes all 3 stages; staged
realistic secret FAILs stage 1 (exit 1); broken type FAILs stage 2;
broken assertion FAILs stage 3; next build exit 0.

package.json

@@ -8,6 +8,7 @@
     "build": "prisma generate && next build",
     "start": "next start",
     "typecheck": "tsc --noEmit",
+    "test": "tsx tests/llm-parse.test.ts",
     "db:generate": "prisma generate",
     "db:push": "prisma db push",
     "db:seed": "tsx prisma/seed.ts",