coder(dispatcher): react to new tasks via LISTEN/NOTIFY, poll as fallback

AFTER INSERT trigger on tasks fires pg_notify('tasks_new'); the dispatcher listens via porsager sql.listen and triggers an immediate poll, with the setInterval poll kept at 2s as a missed-notification safety net. Per-session guard unchanged (no double-dispatch).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/schema.sql

@@ -71,3 +71,22 @@ ALTER TABLE available_agents ADD COLUMN IF NOT EXISTS transport TEXT DEFAULT 'pt
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS mode_id TEXT;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS thinking_option_id TEXT;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS feature_values JSONB;
+
+-- LISTEN/NOTIFY fast path: every tasks INSERT (from any call site — routes,
+-- new_task tool, arena, MCP server) fires pg_notify('tasks_new') in the same
+-- transaction, so the dispatcher reacts immediately instead of waiting for the
+-- fallback poll. Postgres holds the notification until COMMIT, so the listener
+-- always sees the committed row. A trigger covers all insert paths with no
+-- app-code drift. Idempotent: re-applied on every startup.
+CREATE OR REPLACE FUNCTION notify_tasks_new() RETURNS trigger AS $$
+BEGIN
+  PERFORM pg_notify('tasks_new', '');
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS tasks_notify_new ON tasks;
+CREATE TRIGGER tasks_notify_new
+  AFTER INSERT ON tasks
+  FOR EACH ROW
+  EXECUTE FUNCTION notify_tasks_new();