backend-agents D17: host runPrompt → POST /api/turns cutover

[btipling]

Parent: #794
Umbrella: #794
Label: backend-agents
Status: DRAFT
Blocked on: #810 (C16)

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

Slice

app/harness/HarnessHost.tsx runPrompt → POST /api/turns. Capture {runId}, fold turnRunId/turnStatus.

Fail-closed in-canvas Error on durable-path error. No silent /api/agent fallback. /api/agent stays reachable for tests/JSON only.

DoD

• Host starts turns via /api/turns
• Durable-path error → in-canvas Error (EMBER), no silent fallback
• typecheck + vitest + wasm-int lock green
• No Wasm/protocol change

Refs

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