feat(server): trigger agents via @mention in comments

When a user @mentions an agent in any issue's comment, the system now
enqueues a task for that agent. The agent reads the issue context and
replies to the triggering comment thread.

Changes:
- Add shared util.ParseMentions for mention parsing (used by both
  comment handler and notification listeners)
- Add EnqueueTaskForMention to TaskService for explicit agent targeting
- Add on_mention trigger type support in agent trigger config
- Add HasPendingTaskForIssueAndAgent SQL query for per-agent dedup
- Add enqueueMentionedAgentTasks in CreateComment handler

Safety: prevents self-trigger (agent mentioning itself), dedup with
assignee on_comment trigger, terminal issue status check, and per-agent
pending task dedup.

server/pkg/db/queries/agent.sql

@@ -124,6 +124,12 @@ WHERE issue_id = $1 AND status IN ('queued', 'dispatched', 'running');
 SELECT count(*) > 0 AS has_pending FROM agent_task_queue
 WHERE issue_id = $1 AND status IN ('queued', 'dispatched');
 
+-- name: HasPendingTaskForIssueAndAgent :one
+-- Returns true if a specific agent already has a queued or dispatched task
+-- for the given issue. Used by @mention trigger dedup.
+SELECT count(*) > 0 AS has_pending FROM agent_task_queue
+WHERE issue_id = $1 AND agent_id = $2 AND status IN ('queued', 'dispatched');
+
 -- name: ListPendingTasksByRuntime :many
 SELECT * FROM agent_task_queue
 WHERE runtime_id = $1 AND status IN ('queued', 'dispatched')