sync: support post-sync hook in <repo-hooks>

Add support for a new hook type "post-sync" declared in the manifest using
<repo-hooks>. This allows executing a script automatically after a successful
`repo sync`.

This is useful for initializing developer environments, installing project-wide
Git hooks, generating configs, and other post-sync automation tasks.

Example manifest usage:

  <project name="myorg/repo-hooks" path="hooks" revision="main" />
  <repo-hooks in-project="myorg/repo-hooks" enabled-list="post-sync">
    <hook name="post-sync" />
  </repo-hooks>

The hook script must be named `post-sync.py` and located at the root of the
hook project.

The post-sync hook does not block `repo sync`; if the script fails, the sync
still completes successfully with a warning.

Test: Added `post-sync.py` in hook project and verified it runs after `repo sync`

Bug: b/421694721
Change-Id: I69f3158f0fc319d73a85028d6e90fea02c1dc8c8
Signed-off-by: Kenny Cheng <chao.shun.cheng.tw@gmail.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/480581
Reviewed-by: Scott Lee <ddoman@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

docs/repo-hooks.md

@@ -133,3 +133,43 @@ def main(project_list, worktree_list=None, **kwargs):
       kwargs: Leave this here for forward-compatibility.
     """
 ```
+
+### post-sync
+
+This hook runs when `repo sync` completes without errors.
+
+Note: This includes cases where no actual checkout may occur. The hook will still run.
+For example:
+- `repo sync -n` performs network fetches only and skips the checkout phase.
+- `repo sync <project>` only updates the specified project(s).
+- Partial failures may still result in a successful exit.
+
+This hook is useful for post-processing tasks such as setting up git hooks,
+bootstrapping configuration files, or running project initialization logic.
+
+The hook is defined using the existing `<repo-hooks>` manifest block and is
+optional. If the hook script fails or is missing, `repo sync` will still
+complete successfully, and the error will be printed as a warning.
+
+Example:
+
+```xml
+<project name="myorg/dev-tools" path="tools" revision="main" />
+<repo-hooks in-project="myorg/dev-tools" enabled-list="post-sync">
+  <hook name="post-sync" />
+</repo-hooks>
+```
+
+The `post-sync.py` file should be defined like:
+
+```py
+def main(repo_topdir=None, **kwargs):
+    """Main function invoked directly by repo.
+
+    We must use the name "main" as that is what repo requires.
+
+    Args:
+      repo_topdir: The absolute path to the top-level directory of the repo workspace.
+      kwargs: Leave this here for forward-compatibility.
+    """
+```