feat: allow renaming saved workflows from the /workflows navigator

[gr3enarr0w]

Summary

Saved workflows can be saved and deleted from the navigator but not renamed. Renaming is the most common operation after saving.

Proposed design

• Press r on a saved workflow row to rename
• ctx.ui.input({ prompt: 'Rename to:', defaultValue: currentName }) dialog appears
• On valid name: storage.save({ ...existing, name: newName }) + storage.delete(oldName)
• Live re-registers the slash command: pi.unregisterCommand(oldName) + registerSavedWorkflow(...renamed...)

New NavAction

| { kind: 'renameSaved'; savedName: string }

Files to change

• src/workflow-ui.ts — add renameSaved action, handle in openWorkflowNavigator, update keyToAction and footer hints

[QuintinShaw]

Thanks — #164 adds saved-workflow rename for project, legacy, and user records, including immediate live slash-command registration and rollback-safe persistence. It will close this issue when merged.