chore(hooks): pre-push gate, secret scan, pr:evidence drafter

Phase A of the quality-gate rollout — local + pre-push gates that stop
slop before it leaves the laptop. Preventative controls, per the
architect playbook.

- .husky/pre-commit — adds scripts/secret-scan.sh as the first check
  before the existing Biome format/lint. Blocks the commit when a
  likely secret is found in staged additions. Existing Biome behaviour
  is preserved.
- .husky/pre-push (new) — runs `pnpm check` (lint + typecheck + test)
  before the branch leaves the laptop. Mirrors the `check` job in
  .github/workflows/ci.yml so PR-blocking issues surface locally first.
  Bypassable for genuine emergencies with `git push --no-verify`.
- scripts/secret-scan.sh (new) — regex scan over staged additions for
  the common accidents: AWS keys, Anthropic/OpenAI/GitHub/Slack/Google/
  Stripe tokens, JWTs, PEM private key blocks, hardcoded password
  literals. Validated against synthetic leaks of each type. No external
  tool dependency — pure bash + grep, runs in <100ms typical.
- scripts/pr-evidence.sh (new) — drafts a PR_BODY.md from the diff vs
  origin/main: file lists (added/modified/deleted), commit summaries,
  and the QA checklist. Flags screenshots as REQUIRED when the diff
  touches packages/web/src/{components,routes} or packages/desktop.
  Wired as `pnpm pr:evidence` so it runs from anywhere in the
  workspace.
- .gitignore — adds PR_BODY.md so drafts don't get committed by
  accident.

Pre-commit stays fast (<2s typical). Heavy checks (typecheck, full
test run) live in pre-push.

.gitignore

@@ -41,3 +41,6 @@ dist-ssr
 # Tauri
 packages/desktop/src-tauri/target/
 packages/desktop/src-tauri/gen/
+
+# Drafts from scripts/pr-evidence.sh
+PR_BODY.md