chore(githooks): remove pre-push hook (#22956)

## Summary
- remove the `pre-push` git hook script from the repository
- remove the `make pre-push` target and related Makefile documentation
- update contributor and agent docs so they only describe the remaining
`pre-commit` hook

## Validation
- `make pre-commit`
- `git diff --check`

---
_Generated with [`mux`](https://github.com/coder/mux) • Model:
`openai:gpt-5.4` • Thinking: `high`_

docs/about/contributing/CONTRIBUTING.md

@@ -71,7 +71,6 @@ Use the following `make` commands and scripts in development:
 - `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: