backend-agents D18: detach semantics — unmount ≠ abort

[btipling]

Parent: #794
Umbrella: #794
Label: backend-agents
Status: DRAFT
Blocked on: #811 (D17)

Implements one row of the restart umbrella. One issue → one PR → one adversarial review → one merge → close. Do not expand scope.

Slice

lib/detachTurn.ts decideDetach: unmount / switch / New / logout close this reader only. Never abort as "turn over". Never a server cancel.

Explicit no-op when no run id (idle). Stop/Esc still cancels. Host wiring source-lock (4 detach sites / 3 abort sites).

DoD

• Source lock: 4 detachTurn() sites / 3 abort() sites
• Durable run (turnRunId present) → close reader, no abort
• Idle / no run id → no-op
• typecheck + vitest green
• No Wasm/protocol change

Refs

• Parent/umbrella: plan: backend-agents RESTART — client-independent turns (umbrella, ≥20 small issues + handoff) #794