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/killer_plan9.go

@@ -0,0 +1,29 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build plan9
+
+package process
+
+import "os"
+
+// Killer falls back to single-process termination on platforms without a
+// process-group / Job Object tree-kill. The Job Object (Windows) and process
+// group (Unix) based tree-kills live in killer_windows.go / killer_unix.go;
+// here we just kill the direct child, matching the previous default behaviour.
+type Killer struct {
+	p *os.Process
+}
+
+func NewKiller(p *os.Process) (*Killer, error) {
+	return &Killer{p: p}, nil
+}
+
+func (k *Killer) Kill() error {
+	if k == nil || k.p == nil {
+		return nil
+	}
+	return k.p.Kill()
+}
+
+func (k *Killer) Close() error { return nil }