feat: support multiple agents running concurrently on the same issue

- Relax ClaimAgentTask SQL constraint from per-issue to per-(issue, agent)
  serialization, allowing different agents to run in parallel on the same issue
- Update GetActiveTaskForIssue API to return all active tasks (array) instead of
  just the first one
- Refactor AgentLiveCard to render one card per active task, routing WebSocket
  messages by task_id for independent timelines
- Fix shouldEnqueueOnComment to use per-agent dedup so a mentioned agent's
  pending task doesn't block the assigned agent's on_comment trigger

Closes MUL-160

server/pkg/db/queries/agent.sql

@@ -77,10 +77,10 @@ SELECT * FROM agent_task_queue
 WHERE id = $1;
 
 -- name: ClaimAgentTask :one
--- Claims the next queued task for an agent, enforcing per-issue serialization:
--- a task is only claimable when no other task for the same issue is already
--- dispatched or running. This guarantees serial execution within an issue
--- while allowing parallel execution across different issues.
+-- Claims the next queued task for an agent, enforcing per-(issue, agent) serialization:
+-- a task is only claimable when no other task for the same issue AND same agent is
+-- already dispatched or running. This allows different agents to work on the same
+-- issue in parallel while preventing a single agent from running duplicate tasks.
 UPDATE agent_task_queue
 SET status = 'dispatched', dispatched_at = now()
 WHERE id = (
@@ -89,6 +89,7 @@ WHERE id = (
       AND NOT EXISTS (
           SELECT 1 FROM agent_task_queue active
           WHERE active.issue_id = atq.issue_id
+            AND active.agent_id = atq.agent_id
             AND active.status IN ('dispatched', 'running')
       )
     ORDER BY atq.priority DESC, atq.created_at ASC