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_unix.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !windows && !plan9
+
+package process
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+// Killer terminates a started process together with its whole process group,
+// which is the Unix counterpart of the Windows Job Object tree-kill.
+//
+// Background: a process (a step or a post-task script) often launches a process
+// tree (a shell that starts a child which in turn spawns further background
+// processes). The default exec.CommandContext cancellation only kills the
+// direct child, so cancelling left the rest of the tree running. Because those
+// orphans inherited the parent's stdout/stderr pipe, cmd.Wait() also blocked
+// forever and the runner hung.
+//
+// Processes are started with Setpgid (or Setsid for the PTY path, see
+// SysProcAttr), which makes the process the leader of a new process group whose
+// ID equals its PID. Signalling the negative PID delivers to every process
+// still in that group, so we can tear down the whole tree atomically on
+// cancellation, which also closes the inherited pipe handles so cmd.Wait() can
+// return.
+type Killer struct {
+	pgid int
+}
+
+// NewKiller captures the process group of p (an already-started process).
+// Because the process is launched with Setpgid/Setsid, p is a group leader and
+// its PGID equals its PID; children spawned afterwards stay in the same group
+// unless they explicitly create their own.
+func NewKiller(p *os.Process) (*Killer, error) {
+	return &Killer{pgid: p.Pid}, nil
+}
+
+// Kill sends SIGKILL to the entire process group (the process and every
+// descendant that stayed in the group). A missing group (ESRCH) means the
+// processes already exited and is not treated as an error.
+func (k *Killer) Kill() error {
+	if k == nil || k.pgid <= 0 {
+		return nil
+	}
+	if err := syscall.Kill(-k.pgid, syscall.SIGKILL); err != nil && !errors.Is(err, syscall.ESRCH) {
+		return err
+	}
+	return nil
+}
+
+// Close is a no-op on Unix; there is no job handle to release.
+func (k *Killer) Close() error { return nil }