Session composer: start agent sessions from any device

[karngyan]

Why

New-session today spawns a shell in a directory, which is right for a laptop and thin for a phone. The flow people use from a phone is Happy Coder's composer, the benchmark here: pick machine, pick directory, pick agent, set the mode, go. flue already has the new-session route and flue open, this grows them up.

Rough scope

• Machine picker (already have the fleet), directory picker with recents first (daemon remembers recent session cwds per machine), agent picker: claude, codex, plain shell. Only offer agents actually installed on that machine, the daemon can check.
• Resume: offer "continue last conversation in this directory" using the same resume command flue already prints after a reboot brings a session back.
• Optional extras once the basics exist: a worktree checkbox (create a worktree, start the agent inside it), and an initial prompt field so the first message rides along.

Notes

• Keep the fast path fast: enter from the sessions screen, two taps for the common case (same machine, recent directory, same agent as last time).