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.go

@@ -8,23 +8,10 @@ package container
 
 import (
 	"os"
-	"syscall"
 
 	"github.com/creack/pty"
 )
 
-func getSysProcAttr(_ string, tty bool) *syscall.SysProcAttr {
-	if tty {
-		return &syscall.SysProcAttr{
-			Setsid:  true,
-			Setctty: true,
-		}
-	}
-	return &syscall.SysProcAttr{
-		Setpgid: true,
-	}
-}
-
 func openPty() (*os.File, *os.File, error) {
 	return pty.Open()
 }