Proposed sessions

When the agent spots parallel work you didn't ask for (a fix noticed in passing, the natural follow-up), it proposes it with propose_session instead of starting it. Nothing runs until you choose.

In the desktop app, a card offers:

• Start session: runs it as its own chat, on its own git worktree when the project is a repository, under your current approval policy.
• Do it here: asks for it in this chat instead (queued if a turn is running).
• Dismiss: declines it.

In the terminal, /proposals lists the chat's proposals, and /proposals start <n>, /proposals here <n>, and /proposals dismiss <n> act on one.

Limits: a turn can propose at most three sessions, and never the same title twice in one chat.