backend-agents C16: GET /api/turns/:runId/stream attach/reconnect proof

[btipling]

Parent: #794
Umbrella: #794
Label: backend-agents
Status: DRAFT
Blocked on: #809 (C15)

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

Slice

GET /api/turns/:runId/stream?startIndex= — getReadable({startIndex}) with startIndex=0 / MID replay.

abort ≠ cancel. getRun is status truth. Producer closed the writable.

DoD

• startIndex=0 replays from the first chunk; MID resumes
• Client abort does not cancel the run
• typecheck + vitest green

Out of scope

Host attach UI (E19). Stop → server cancel (G22).

Refs

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