Add context-aware items to the command palette

When opened with Ctrl-K, the palette now prepends entries for whatever
is currently focused:

- Focused scratchpad: Delete / Rename (inline form) / Edit (fire-and-
  forget zed launch with stdio detached so the TUI is not suspended).
- Focused agent: Rename (inline form) / Close.
- Focused process: Rename / Delete (drops the entry; SIGKILL if alive)
  / Stop (SIGTERM, keep entry) / Restart (same argv).

The rename UX is a single-field inline form that mirrors the existing
spawn-process form, so the modal-input contract is unchanged.
scratchpad.Store grows Delete / Rename / Path so the palette can act
on a pad file by name. focusedPad is plumbed onto uiState ahead of the
scratchpad-focus UI work; until that lands it stays empty and the
scratchpad-context entries simply never surface.

Tested with palette_context_test.go and a new rename_process_via_palette
harness scenario.

internal/scratchpad/scratchpad.go

@@ -148,6 +148,52 @@ func (s *Store) Append(name, content string) error {
 	return err
 }
 
+// Delete removes the scratchpad file. Missing files are reported as
+// errors; callers that want "delete if exists" can ignore os.ErrNotExist.
+func (s *Store) Delete(name string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	p, err := s.safePath(name)
+	if err != nil {
+		return err
+	}
+	return os.Remove(p)
+}
+
+// Rename moves a scratchpad file to a new name within the same project
+// directory. Returns os.ErrExist if newName already exists; the caller
+// is expected to surface that to the user rather than clobber.
+func (s *Store) Rename(oldName, newName string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	src, err := s.safePath(oldName)
+	if err != nil {
+		return err
+	}
+	dst, err := s.safePath(newName)
+	if err != nil {
+		return err
+	}
+	if src == dst {
+		return nil
+	}
+	if _, err := os.Stat(dst); err == nil {
+		return fmt.Errorf("scratchpad: %q already exists", newName)
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+	return os.Rename(src, dst)
+}
+
+// Path returns the absolute path of a scratchpad file. The file does
+// not need to exist; callers like "Edit scratchpad" rely on this to
+// hand the path to an external editor.
+func (s *Store) Path(name string) (string, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.safePath(name)
+}
+
 func (s *Store) safePath(name string) (string, error) {
 	if name == "" || strings.ContainsAny(name, "/\\") || name == "." || name == ".." {
 		return "", errors.New("scratchpad: invalid name")