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_plan9.go

@@ -0,0 +1,17 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build plan9
+
+package process
+
+import "syscall"
+
+// SysProcAttr returns the platform attributes used to start a process. Plan 9
+// has no process-group tree-kill (see Killer), so we only request a new rfork
+// note group here.
+func SysProcAttr(cmdLine string, tty bool) *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		Rfork: syscall.RFNOTEG,
+	}
+}