fix(execution): opencode serve protocol — object-shaped model post + settled terminal stream

[robertelee78]

Summary

Two bugs in the opencode execution adapter, found by the #76 three-host live smoke where the opencode leg could not be exercised:

1. Every configured-model worker 400'd. The adapter posted model as a bare string to POST /session/:id/prompt_async, but opencode serve's schema expects {providerID, modelID} or null:

   {"name":"BadRequest","data":{"message":"Expected object | null, got \"opencode/kimi-k3\"\n  at [\"model\"]","kind":"Payload"}}
   

   provider/model strings are now split on the first slash into the schema object (model ids may contain slashes); a model with no provider prefix is omitted so the server's own configured default applies.

2. The CLI crashed after printing the failure verdict. When the prompt post threw, launch() stopped the owned server but left the terminal SSE promise unconsumed; its socket-close rejection surfaced as an unhandled rejection (TypeError: terminated, UND_ERR_SOCKET) and killed the process after the run verdict. A no-op second consumer is attached at creation — observe() still sees genuine terminal rejections; the owned server still gets SIGTERM.

Verification

• Regression tests: serve-shaped model posts (incl. slash-containing model ids like openrouter/z-ai/glm-5.2, and the bare-id omission), and a 400-prompt teardown asserting no unhandled rejection while the owned server still receives SIGTERM.
• pnpm run check exit 0 (1049 kit + 222 cjs tests), pnpm run test:surface exit 0 (25).
• Live proof: with this fix, the opencode leg of the feat: generalize host routing and add OpenCode as a routable execution host #76 smoke completes — ak run feature with all four workers routed to opencode (kimi-k3) succeeded end-to-end, produced the sandboxed repository mutation (hello.mjs + tests/), and emitted normalized terminal evidence (observed model kimi-k3, provider opencode, per-worker durations). Smoke detail posted on feat: generalize host routing and add OpenCode as a routable execution host #76.

Refs #76