F10: Agent session (t3code instance) as draggable — move threads between projects #13

Summary

Define a "t3code instance" as a Thread with an active Agent Session, and allow the user to move it between Projects within the same Client by dragging it in the sidebar.

Definition

A t3code instance is a Thread where `session.status` is not in `{stopped, null}`. The sidebar differentiates these from inactive Threads with a visual activity indicator.

Scope

• Add new command: `thread.project.reassign` with `{threadId, targetProjectId}`
• Add new event: `thread.project.reassigned` with `{threadId, previousProjectId, targetProjectId}`
• Decider validates:
  • Source and target Projects belong to the same Client
  • Source and target Projects share the same `workspaceRoot` (agent context is workspace-bound)
• Projector updates Thread's `projectId`
• All associated Terminals move with the Thread (they reference threadId, not projectId directly)

Sidebar UI

• Thread items with active agent sessions show an activity indicator (pulsing dot or similar)
• Thread items are draggable within the sidebar
• Valid drop zones: Projects within the same Client that share the same `workspaceRoot`
• Invalid drops (different Client, different workspaceRoot): show rejection feedback with explanation

Edge cases

• Thread with active turn during drag: move permitted, turn continues
• Thread with no agent session: move still permitted (regular Thread move)
• Drop onto Project in a different Client: rejected ("Move Projects between Clients, not Threads")

Key files to modify

• `packages/contracts/src/orchestration.ts` — new command/event schemas
• `apps/server/src/orchestration/decider.ts` — validation logic
• `apps/server/src/orchestration/projector.ts` — update Thread's projectId
• `apps/web/src/components/Sidebar.tsx` — Thread drag + validation + drop zones

Dependencies

• F2 (Client-Project binding — need to know which Client a Project belongs to)

Acceptance criteria

• Threads with active sessions show visual indicator
• Can drag a Thread to another Project in same Client (same workspaceRoot)
• Agent session continues running after move
• Terminals associated with the Thread move with it
• Cross-Client Thread moves rejected with explanation
• Different-workspaceRoot moves rejected with explanation
• Threads without agent sessions can also be moved