inject a mid-turn message instead of replacing the turn

Phase C server half, and a real flaw in phase B.

On the subprocess path a second message could only supersede — kill the process, start again,
lose the turn — because opencode run has no input channel. The serve takes another prompt
into the running turn, so a message arriving mid-turn is handed over with delivery steer and
the existing turn is left exactly as it is.

Keeping the same turn object is the load-bearing part. Phase B retired it and registered a
replacement, which stops officer routing events the serve is still producing while the serve
carries on regardless: output goes nowhere and the turn looks hung.

Verified end to end through the chat socket — sent a count to 50, injected a change of plan
eight seconds in, and BANANA INJECTED came back inside the same turn with deltas streaming
throughout.

No client change was needed. Officer composer already sends while generating; the difference
is only what the sidecar does with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/opencode-serve-migration-plan.md

@@ -58,6 +58,19 @@ Each of these cost time to find. None is in the API docs.
 5. **The event names are `session.next.*`** — `step.started`, `text.started`, `text.ended`,
    `tool.called`, `tool.success`, `step.ended`, `step.failed`. Not the shapes `mapRunLine` handles.
 
+## Status, 2026-08-10
+
+- **Phase A — done.** `serve-events.ts` + tests, fixtures captured from real turns.
+- **Phase B — done, behind `OPENCODE_TURNS=serve` (default: subprocess).** `serve-runner.ts`. Verified
+  end to end through the chat socket: tool call, tool result, **3 streaming deltas**, text, cost. Stop is
+  an interrupt and the session survives it.
+- **Phase C — server half done.** A message sent while a turn runs is injected with `delivery: "steer"`
+  into the RUNNING turn, verified end to end. No client change was needed: officer's composer already
+  sends mid-turn, and the subprocess path was superseding where the serve steers.
+- **Phase D — not started, deliberately.**
+
+Not yet lived with. Nothing here has run a real conversation with a person at the other end.
+
 ## Shape of the work
 
 **Phase A — read the stream without depending on it.** Add a serve-based reader alongside the existing