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/process/sysprocattr_unix.go

@@ -0,0 +1,24 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !windows && !plan9
+
+package process
+
+import "syscall"
+
+// SysProcAttr returns the platform attributes used to start a process so that a
+// Killer can later tear down its whole process tree. On Unix the process becomes
+// the leader of a new process group (or session, for the PTY path), so a
+// signal to the negative PID reaches every descendant that stayed in the group.
+func SysProcAttr(_ string, tty bool) *syscall.SysProcAttr {
+	if tty {
+		return &syscall.SysProcAttr{
+			Setsid:  true,
+			Setctty: true,
+		}
+	}
+	return &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+}