fix(server): improve comment trigger logic for agent execution

- Add thread-aware on_comment suppression: when a member replies in a
  thread started by another member without @mentioning the assignee
  agent, the on_comment trigger is now suppressed. This fixes the bug
  where member-to-member conversations incorrectly triggered the
  assigned agent.

- Add terminal status check to on_mention: enqueueMentionedAgentTasks
  now skips done/cancelled issues, consistent with on_comment behavior.

- Write explicit default triggers on agent creation: new agents get
  [on_assign, on_comment, on_mention] all enabled, instead of relying
  on null/empty = all enabled. Existing agents with empty triggers
  still work via backwards-compat fallback in agentHasTriggerEnabled.

- Consolidate trigger check logic into shared agentHasTriggerEnabled
  helper, fixing inconsistency where empty [] was handled differently
  by isAgentTriggerEnabled (returned false) vs isAgentMentionTriggerEnabled
  (returned true).

- Add documentation comments explaining the intentional status gate
  difference: on_assign fires only for todo (start new work), while
  on_comment fires for any non-terminal status (conversational).

server/internal/handler/agent.go

@@ -275,7 +275,7 @@ func (h *Handler) CreateAgent(w http.ResponseWriter, r *http.Request) {
 
 	triggers, _ := json.Marshal(req.Triggers)
 	if req.Triggers == nil {
-		triggers = []byte("[]")
+		triggers = defaultAgentTriggers()
 	}
 
 	agent, err := h.Queries.CreateAgent(r.Context(), db.CreateAgentParams{