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>

act/container/util_windows.go

@@ -7,13 +7,8 @@ package container
 import (
 	"errors"
 	"os"
-	"syscall"
 )
 
-func getSysProcAttr(cmdLine string, tty bool) *syscall.SysProcAttr {
-	return &syscall.SysProcAttr{CmdLine: cmdLine, CreationFlags: syscall.CREATE_NEW_PROCESS_GROUP}
-}
-
 func openPty() (*os.File, *os.File, error) {
 	return nil, nil, errors.New("Unsupported")
 }