fix(daemon): prevent stuck tasks from blocking queue and add concurrent execution

- Expand FailAgentTask SQL to accept dispatched OR running status
- Add FailStaleTasks server-side sweeper (dispatched >5min, running >2.5h)
- Fix daemon handleTask to fail tasks on all error paths (StartTask, CompleteTask)
- Make daemon poll loop concurrent with semaphore (default 20 parallel tasks)
- Raise default agent max_concurrent_tasks from 1 to 6 (migration 023)
- Add --max-concurrent-tasks CLI flag and MULTICA_DAEMON_MAX_CONCURRENT_TASKS env

server/pkg/db/queries/agent.sql

@@ -90,9 +90,19 @@ LIMIT 1;
 -- name: FailAgentTask :one
 UPDATE agent_task_queue
 SET status = 'failed', completed_at = now(), error = $2
-WHERE id = $1 AND status = 'running'
+WHERE id = $1 AND status IN ('dispatched', 'running')
 RETURNING *;
 
+-- name: FailStaleTasks :many
+-- Fails tasks stuck in dispatched/running beyond the given thresholds.
+-- Handles cases where the daemon is alive but the task is orphaned
+-- (e.g. agent process hung, daemon failed to report completion).
+UPDATE agent_task_queue
+SET status = 'failed', completed_at = now(), error = 'task timed out'
+WHERE (status = 'dispatched' AND dispatched_at < now() - make_interval(secs => @dispatch_timeout_secs::double precision))
+   OR (status = 'running' AND started_at < now() - make_interval(secs => @running_timeout_secs::double precision))
+RETURNING id, agent_id, issue_id;
+
 -- name: CountRunningTasks :one
 SELECT count(*) FROM agent_task_queue
 WHERE agent_id = $1 AND status IN ('dispatched', 'running');