build: add pre-commit/push hooks mirroring CI checks (#22705)

This change adds git hooks and Makefile targets that mirror CI required
checks locally, catching issues before they reach CI.

This is for use by AI agents (documented in AGENTS.md).

- **pre-commit** (every commit): gen, fmt, lint, typos, slim binary
  build. Fast checks without Docker or Playwright.
- **pre-push** (before push): full CI suite including site build, tests,
  sqlc-vet, offlinedocs.
  
To use:

```sh
git config core.hooksPath scripts/githooks
```

Works in worktrees (where `.git` is a file). Bypass with `--no-verify`.

scripts/githooks/pre-commit

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+#
+# Pre-commit hook that runs CI-equivalent checks locally.
+# Runs `make pre-commit` (gen, fmt, lint, typos, build) which
+# catches most CI failures without needing Docker or Playwright.
+# The full CI suite (including tests) runs via the pre-push hook.
+#
+# Installation (worktree-compatible):
+#
+#   git config core.hooksPath scripts/githooks
+#
+# Bypass: git commit --no-verify
+
+set -euo pipefail
+
+cd "$(git rev-parse --show-toplevel)"
+unset GIT_DIR
+
+exec make pre-commit