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

apps/web/shared/api/client.ts

@@ -379,7 +379,7 @@ export class ApiClient {
     return this.fetch(`/api/agents/${agentId}/tasks`);
   }
 
-  async getActiveTaskForIssue(issueId: string): Promise<{ task: AgentTask | null }> {
+  async getActiveTasksForIssue(issueId: string): Promise<{ tasks: AgentTask[] }> {
     return this.fetch(`/api/issues/${issueId}/active-task`);
   }