feat(scripts/githooks): restore pre-push hook with allowlist (#22980)

The pre-push hook was removed in #22956. This restores it with a
reduced scope (tests + site build) and an allowlist so it only runs
for developers who opt in.

Two opt-in mechanisms:

- git config coder.pre-push true (local, not committed)
- CODER_WORKSPACE_OWNER_NAME allowlist in the hook script

git config takes priority and also supports explicit opt-out for
allowlisted users (git config coder.pre-push false).

Refs #22956

---------

Co-authored-by: Cian Johnston <cian@coder.com>

scripts/githooks/pre-commit

@@ -3,6 +3,7 @@
 # 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.
+# Heavier checks (tests, site build) run via the pre-push hook.
 #
 # Installation (worktree-compatible):
 #