fix(grok): surface a dead opencode-serve clearly instead of a zombie chat (M2)

If `opencode serve` died after the session opened, every subsequent turn failed
with an opaque httpx connection error while the container lingered. send() now
detects the exited subprocess (returncode set) and yields a clear error chunk +
turn_end so the panel shows a real "session ended — start a new chat" message;
the idle watchdog / a human reap then tears the container down.

tests/unit/agent_sdk/test_opencode_session.py

@@ -9,7 +9,9 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
+import pytest
 from roboco.agent_sdk.opencode_session import (
+    OpencodeServeSession,
     _extract_session_id,
     _message_error,
     normalize_opencode_message,
@@ -51,6 +53,21 @@ def test_fenced_draft_in_text_becomes_draft_chunk() -> None:
     assert draft.data["title"] == "Add login"
 
 
+@pytest.mark.asyncio
+async def test_send_on_dead_serve_yields_clear_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    # A crashed `opencode serve` must surface a clear error + end the turn, not
+    # hang the chat with opaque connection errors while the container zombies.
+    sess = OpencodeServeSession()
+    monkeypatch.setattr(sess, "_session_id", "ses-1")
+    monkeypatch.setattr(sess, "_client", object())  # unused: dead-proc guard wins
+    monkeypatch.setattr(sess, "_proc", type("P", (), {"returncode": 1})())
+    chunks = [c async for c in sess.send("hi")]
+    assert [c.kind for c in chunks] == ["error", "turn_end"]
+    assert "exited" in chunks[0].text
+
+
 def test_propose_draft_tool_part_becomes_draft_chunk() -> None:
     # The intake-tools.js propose_draft tool call (its input nested under
     # `draft`) is intercepted into a draft chunk — NOT rendered as a tool_use —