build: address review on the pre-push hook and hooks-dir resolution

resolveGitHooksDir now returns a directory only when it is the repo's own
<git-common-dir>/hooks (compared on canonical paths), so a core.hooksPath
elsewhere, global or repo-local, is never written to by postinstall, while a
core.hooksPath pointing back at the repo's own .git/hooks still resolves.

The pre-push hook skips with a one-line notice when a pushed ref is not the
checked-out HEAD (tags peeled) or when git status shows uncommitted or
untracked changes under a path the checks read (src, config, scripts, test,
package.json, package-lock.json, install.sh), since the checks read the
working tree rather than the pushed commit.

Also: honest timing (~10-40s instead of ~15s), CLAUDE.md Session Safety note
on CODEMAN_SKIP_PREPUSH for another session's WIP, 14 (not 9) Playwright
tests, and a note that the browser-excludes check only sees direct imports.

.github/CONTRIBUTING.md

@@ -35,7 +35,7 @@ npm run check:frontend-syntax   # syntax-checks the plain-JS frontend modules
 npm run check:browser-excludes  # every browser-driven test is kept out of `npm test`
 ```
 
-`npm install` also installs a `pre-push` git hook that runs these static checks (~15s) and blocks the push if one fails. Skip it once with `CODEMAN_SKIP_PREPUSH=1 git push`; it never replaces a `pre-push` hook of your own.
+`npm install` also installs a `pre-push` git hook that runs these static checks (about 10-40s, machine-dependent) and blocks the push if one fails. It skips itself when you push something other than the checked-out HEAD, or when the tree has uncommitted changes the checks would read. Skip it once with `CODEMAN_SKIP_PREPUSH=1 git push`; it never replaces a `pre-push` hook of your own.
 
 ### Tests