feat: Add optional runner.post_task_script hook after task cleanup (#1026)

- Adds `runner.post_task_script` and `runner.post_task_script_timeout` (default `5m`) to run a host executable after each task’s built-in cleanup (post-steps, container teardown, bind-workdir removal).
- Stops task heartbeats via `Reporter.StopHeartbeats()` while the script runs so Gitea won’t assign overlapping work; the final task acknowledgement still happens in `reporter.Close()`.
- Script output goes to the runner process log; non-zero exits are warned only and do not change the job result.
- Documents lifecycle, offline behavior, timeouts, and Windows limits (`.ps1` not supported yet) in `docs/post-task-script.md`.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1026
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

internal/pkg/config/config.example.yaml

@@ -83,6 +83,21 @@ runner:
   # terminal; tools like `docker build` emit redrawing progress frames into the captured log
   # when a TTY is present.
   allocate_pty: false
+  # Optional executable on the host, run once after each task's built-in cleanup
+  # (post-steps, container teardown, bind-workdir removal). Additive only.
+  #
+  # IMPORTANT: While this script runs the runner stops task heartbeats and stays
+  # offline from Gitea's perspective until the script exits. A script that never
+  # returns blocks new work until post_task_script_timeout kills it (default 5m).
+  # Keep scripts short; set post_task_script_timeout to a safe upper bound.
+  #
+  # Output -> runner process log (not the job log). Non-zero exit -> warning only.
+  # Windows: use .exe, .bat, or .cmd. PowerShell (.ps1) is not supported yet as
+  # the configured path; wrap PowerShell commands in a .cmd file instead.
+  # Full guide: docs/post-task-script.md
+  post_task_script: ''
+  # Hard limit on post_task_script runtime. Default if omitted: 5m.
+  post_task_script_timeout: 5m
 
 cache:
   # Enable the built-in cache server (used by actions/cache and similar actions).