v0.9.2 — OpenCode G2 empty-loop: response via 'message' preview key

Fix — OpenCode G2 empty-response-loop (part 1 of 2)

Healthy OpenCode tickets were moved to Blocked with
empty_response_loop budget exceeded (consecutive_empty_turns=3, threshold=3)
despite producing real output every turn.

• OpenCode delivered its turn response only under the result / response keys of
  the EVENT_TURN_COMPLETED payload — keys _preview_from_payload never reads — so
  current_turn_message stayed blank and the G2 guard counted every turn empty.
• Fix: the turn-completed payload now also carries message, so a productive turn
  populates the preview and resets the empty-turn counter. A genuinely empty turn
  still counts, so real empty-loop detection is preserved. EMPTY_TURN_LOOP_THRESHOLD
  unchanged at 3. Backend-local; no shared-code change.

Necessary but not sufficient on its own for OpenCode >= 1.x — the response text
itself wasn't being parsed. See v0.9.3.

Full test suite green (1058 passed).