[UncertaintyDeterminesYou4ndMe]

Adds planning mode and a plan approval card to the composer. Desktop and terminal share the same code, as agreed in #6078.

Behaviour

• Plan toggle. A "Plan" button next to the sandbox selector, plus a /plan command, switches the conversation between agent and planning mode (metadata.collaborationMode).
  • It only appears when the Agent supports Plans.
  • On a new conversation, the mode is applied right after the session is created. If the Host answers tools_unavailable, the toggle hides and the draft goes back to agent mode.
• When the toggle is disabled. It is disabled while a turn is running, and while an approved Plan is waiting for admission or running. Switching mode bumps the boundary revision, and that would interrupt the execution (scheduler_authorization_changed).
• Plan panel. "Plan" is the first panel in the composer details. It opens by itself when a new proposal, or a new revision of one, arrives.
  • Proposal: title, overview, numbered steps with their files, and risks.
  • Buttons on a proposal:
    • Approve and execute: the approval uses the conversation's current workspace, sandbox, binding, model and thinking level.
    • Request revision: switches back to planning mode first, because only planning mode lets the model submit the revised Plan.
    • Abandon proposal.
  • Execution: the steps' progress.
    • While it runs, the card offers Cancel Plan.
    • When it is interrupted, the card offers Resume Plan and Cancel Plan.
• Approving does not change the mode. The Host runs an approved Plan in the background as agent work whatever mode the conversation is in, so the conversation stays in planning mode.
• Where errors appear. Mode and Plan errors show in the plan panel or the toggle tooltip, not in the sandbox menu.
• Control journal. changePlan is added to the client's control methods, so Plan actions go through the journal like other mutations.

Notes

• Host gap, not addressed here: the execution's opening message renders as a user bubble showing raw JSON. I'll raise it in [DISCUSS] Rust execution and GPUI desktop rewrite #6078.

Tests

• terminal::components::plan_proposal_opens_for_approval_and_then_shows_its_progress covers the shared card in the terminal.
• terminal::desktop_new_session::desktop_plans_first_then_runs_the_approved_plan drives the desktop against a real Host. It checks that:
  • planning mode offers SubmitPlan;
  • approval runs the steps with update_plan and without SubmitPlan;
  • the Plan completes;
  • the conversation is still in planning mode afterwards.

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code wrote the change and this description; its commit carries Generated-by: Claude Code.