Offline CLI and HTTP project removal dropped force and left native v2 threads
behind. Move the nonempty-project guard and durable child cleanup into the
shared project service, and forward force from CLI, HTTP, and WebSocket calls.

Reuse the thread deletion planner and command lock, hydrate migrated history
before attachment cleanup, and validate child receipts. Commit the project
deletion after its children so failed cleanup can be retried safely.

Validation covers CLI deletion with active and archived threads, missing
workspaces, durable cleanup, partial retries, migrated attachments, receipt
collisions, and concurrent thread updates. Scoped server tests, typecheck, and
lint pass.

Implemented with Codex (GPT-6).