fix: stop host-mode jobs from leaking processes on Windows (#1080)

## Problem

On host-mode Windows runners, a job can leave processes running after it finishes,
and those leftovers hold file handles that block deletion of the workspace.

Today a step's tree is only torn down when the step is *cancelled*
(`process.Killer`). A step that completes leaves whatever it spawned alive, and
the existing workspace scan in `terminateRunningProcesses` misses two shapes of
leftover: orphans whose parent already exited (no tree to walk, and their
executable often lives outside the workspace), and processes that merely *run in*
the workspace but reference no path from it — `Win32_Process` exposes no working
directory, so the scan cannot match them.

## Solution

Two additions in `internal/pkg/process`, both no-ops outside Windows:

- **`process.Group`** — a job-scoped Windows Job Object created with
  `JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE`. Step processes are assigned to it before
  they get their own `Killer`, so the step's job nests inside the job's:
  cancellation still kills exactly the step's tree, while `Remove` closing the
  group makes the kernel terminate everything still assigned, whatever its
  parentage. The kernel also drops the handle when the runner exits, so a crashed
  runner cannot strand processes.

- **`process.KillProcessesWithCWDUnder`** — a best-effort net for processes that
  never joined the job (started via a service or scheduled task). It reads each
  process's working directory from its PEB and terminates those under a workspace
  dir. Processes it cannot open are skipped.

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1080
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/process/group_others.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build !windows
+
+package process
+
+import "os"
+
+// Group is a no-op outside Windows: process groups already reclaim a step's
+// tree, and an orphan does not block workspace deletion the way an open Windows
+// handle does.
+type Group struct{}
+
+// NewGroup returns a Group that owns nothing.
+func NewGroup() (*Group, error) {
+	return &Group{}, nil
+}
+
+// Assign does nothing.
+func (g *Group) Assign(_ *os.Process) error {
+	return nil
+}
+
+// Close does nothing.
+func (g *Group) Close() error {
+	return nil
+}