Workflow round 2: sign-in status, project defaults, queue + plan upgrades, lazy Git pane, terminal cleanup #1501

This follows #1493.

G1. Provider sign-in status and first-run discovery. Settings › Agents only checks that the binary is on PATH.

• Probe each CLI's login state and show "signed out" in the picker and the Agents pane, with a Sign in action that opens the login command in the new terminal.
• Onboarding lists repos the user worked in recently, found with the existing CLI-session scanners, for one-step add.

G2. Per-project defaults.

• Provider, model, effort and permission mode per project, with a sticky "last used" for new threads.

G3. Queue and plan upgrades.

• Queued items drain as separate turns instead of one combined prompt.
• "Steer now" on a queued item.
• Drag to reorder.
• Plan card: a feedback field on Keep planning, Implement in a new thread, and Save plan to file.

G4. Bundle and cleanup.

• Lazy-load the Git/Changes pane to win back the main-chunk growth from Workflow round: review, PR loop, composer, compaction, real terminal #1493.
• Deleting a thread kills its terminals and removes their saved scrollback (rest of P1: Bind terminal sessions to the current worktree and clean up removed threads #1183).