[Feature] `pv rename <id> "new title"` - rename a prompt

[kumar-ayan]

Currently there's no way to rename a prompt after saving it.

What to build:

• pv rename 3 "Better summarizer prompt" updates the title
• Confirm before renaming: "Rename to 'Better summarizer prompt'? [y/N]"
• Show success: "Prompt [Feature] pv rename <id> "new title" - rename a prompt #3 renamed successfully"

Files to touch: src/commands/ (new renameCmd.ts), src/db/queries.ts