Unify startup: show resumable sessions inline with the new-session prompt

On startup, show recent sessions as the default landing screen instead of a blank prompt. Typing a new message would start a new session (the current default path), but you'd also see your recent sessions right there to resume.

Currently the flows are separate:

• claude → blank prompt (new session)
• claude --resume → session picker

Merging these into a single startup UX would reduce friction for resuming work.

Rough idea

• Launch claude → see a list of recent sessions
• Select one to resume, or just start typing to begin a new session
• Essentially the --resume picker with an inline prompt for new sessions

Not a big deal if this doesn't fit the vision — just a thought.