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_windows.go

@@ -0,0 +1,53 @@
+//go:build windows
+
+package hooks
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"os/exec"
+
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// runHook executes the hook and enforces a timeout on Windows.
+// Windows lacks Unix-style process groups; on timeout we best-effort kill
+// the started process. Descendant processes may survive if they detach,
+// but this preserves previous behavior while keeping tests green on Windows.
+func (r *Runner) runHook(hookPath, event string, issue *types.Issue) error {
+	ctx, cancel := context.WithTimeout(context.Background(), r.timeout)
+	defer cancel()
+
+	issueJSON, err := json.Marshal(issue)
+	if err != nil {
+		return err
+	}
+
+	cmd := exec.CommandContext(ctx, hookPath, issue.ID, event)
+	cmd.Stdin = bytes.NewReader(issueJSON)
+
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+
+	done := make(chan error, 1)
+	go func() {
+		done <- cmd.Wait()
+	}()
+
+	select {
+	case <-ctx.Done():
+		if cmd.Process != nil {
+			_ = cmd.Process.Kill()
+		}
+		<-done
+		return ctx.Err()
+	case err := <-done:
+		return err
+	}
+}