docs: add AGit pull request instructions

CONTRIBUTING.md

@@ -20,3 +20,17 @@ cargo deny check advisories
   first-party endpoints lives in `crates/codegen/kigi-env`.
 - Upstream syncs are evaluated commit-by-commit and cherry-picked manually;
   see the PRD §8.1 policy (`git range-diff` records the adopted set).
+
+## Opening a pull request on git.zacharyzhang.com
+
+Forking is disabled on this server. Open pull requests with AGit instead:
+
+1. Register an account at https://git.zacharyzhang.com
+2. Clone, commit your changes, then push to a `refs/for/<target-branch>` ref:
+
+   ```bash
+   git clone https://git.zacharyzhang.com/ZacharyZhang-NY/Kigi-CLI.git
+   git push origin HEAD:refs/for/main -o topic=<short-name> -o title="<PR title>" -o description="<PR body>"
+   ```
+
+3. To update the pull request, push again with the same `topic`.