Install Claude Code integration: menubar item backed by a CLI diff-and-confirm flow

[arzafran]

From TODO.md (Claude Code Integration).

Menubar item "Install Claude Code integration":

• opens a new terminal
• shows the diff it wants to make to the user's config (claude.json, and later codex/opencode configs)
• asks for y to confirm
• implemented in the programa CLI, the menubar item just triggers the CLI command

This is the front door of the agent story. Make it boring, transparent, and reversible.