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`.

docs/about/contributing/CONTRIBUTING.md

@@ -70,6 +70,14 @@ Use the following `make` commands and scripts in development:
 - `make build` compiles binaries and release packages
 - `make install` installs binaries to `$GOPATH/bin`
 - `make test`
+- `make pre-commit` runs gen, fmt, lint, typos, and builds a slim binary
+- `make pre-push` runs the full CI suite including tests
+
+Install the git hooks to run these automatically:
+
+```sh
+git config core.hooksPath scripts/githooks
+```
 
 ### Running Coder on development mode