hooks: harden timeout kill and add platform split

- Move runHook into build-tagged implementations (unix/windows) to keep unix syscalls off Windows builds.
- In unix implementation, guard nil Process, return wrapped kill errors except ESRCH, and document linkage to TestRunSync_KillsDescendants.
- On Windows, best-effort kill on timeout retains prior behavior.
- In tests, move testing.Short earlier and keep descendant-kill coverage on Linux.

internal/hooks/hooks_test.go

@@ -260,6 +260,10 @@ func TestRunSync_KillsDescendants(t *testing.T) {
 		t.Skip("TestRunSync_KillsDescendants requires Linux /proc")
 	}
 
+	if testing.Short() {
+		t.Skip("Skipping long-running descendant kill test in short mode")
+	}
+
 	tmpDir := t.TempDir()
 	hookPath := filepath.Join(tmpDir, HookOnCreate)
 	pidFile := filepath.Join(tmpDir, "child.pid")
@@ -279,10 +283,6 @@ func TestRunSync_KillsDescendants(t *testing.T) {
 	}
 	issue := &types.Issue{ID: "bd-test", Title: "Test"}
 
-	if testing.Short() {
-		t.Skip("Skipping long-running descendant kill test in short mode")
-	}
-
 	err := runner.RunSync(EventCreate, issue)
 	if err == nil {
 		t.Fatal("Expected RunSync to return an error on timeout")