fix(daemon): reply under the specific mentioning comment per thread (MUL-4348 review nit #1)

Non-trigger threads previously replied under the thread root, while the
trigger's thread replied under the trigger comment — asymmetric, and it put
the answer at the top of the thread instead of next to the actual question
when the mention was a mid-thread reply. Reply under the NEWEST triggering
comment in each thread instead (inputs are chronological, so last-write-wins
per thread), making every thread consistent and nesting each answer beside
its question.

Co-authored-by: multica-agent <github@multica.ai>

server/internal/daemon/prompt.go

@@ -241,11 +241,14 @@ func buildCommentPrompt(task Task, provider string) string {
 // threads yield one group each so the agent replies inside each thread instead
 // of merging them into one blob (MUL-4348).
 //
-// The trigger's own thread replies under the trigger comment itself, matching
-// the long-standing single-thread behaviour (--parent = trigger comment id);
-// every other thread replies under its root comment id. Returns nil when there
-// is no trigger or only a single distinct thread — the caller then keeps the
-// existing single-parent reply path unchanged.
+// The reply for each thread targets the NEWEST comment that triggered this run
+// in that thread (coalesced comments arrive oldest-first and the trigger is the
+// newest overall, so a simple last-write-wins yields the newest per thread).
+// That nests the answer next to the most recent question in the thread rather
+// than at the thread root, and makes the trigger's own thread (--parent =
+// trigger comment) consistent with every other thread instead of a special
+// case. Returns nil when there is no trigger or only a single distinct thread —
+// the caller then keeps the existing single-parent reply path unchanged.
 func commentReplyThreads(task Task) []execenv.ThreadReplyTarget {
 	if task.TriggerCommentID == "" {
 		return nil
@@ -261,6 +264,9 @@ func commentReplyThreads(task Task) []execenv.ThreadReplyTarget {
 
 	order := make([]string, 0, len(task.CoalescedComments)+1)
 	parentByThread := make(map[string]string, len(task.CoalescedComments)+1)
+	// note records first-seen order but lets the newest comment win the reply
+	// target: inputs are chronological (coalesced oldest-first, trigger last),
+	// so the last write for a thread is its newest triggering comment.
 	note := func(threadID, parentID string) {
 		if _, ok := parentByThread[threadID]; !ok {
 			order = append(order, threadID)
@@ -268,16 +274,14 @@ func commentReplyThreads(task Task) []execenv.ThreadReplyTarget {
 		parentByThread[threadID] = parentID
 	}
 
-	// Coalesced (older) comments first: each replies under its own thread root.
+	// Coalesced (older) comments first: reply under the specific comment that
+	// mentioned the agent, not the thread root, so a mid-thread mention gets its
+	// answer next to the question.
 	for _, cc := range task.CoalescedComments {
-		tid := threadKey(cc.ThreadID, cc.ID)
-		if _, ok := parentByThread[tid]; ok {
-			continue // thread already represented; keep its first parent
-		}
-		note(tid, tid)
+		note(threadKey(cc.ThreadID, cc.ID), cc.ID)
 	}
-	// The newest trigger last: its thread replies under the trigger comment,
-	// overriding any coalesced comment that shared the trigger's thread.
+	// The newest trigger last: it always wins its own thread's reply target,
+	// overriding any earlier coalesced comment that shared the trigger's thread.
 	note(threadKey(task.TriggerThreadID, task.TriggerCommentID), task.TriggerCommentID)
 
 	if len(order) <= 1 {

server/internal/daemon/prompt_test.go

@@ -741,8 +741,11 @@ func TestCommentReplyThreadsGrouping(t *testing.T) {
 		if got["thread-A"] != "c3" {
 			t.Errorf("trigger thread parent = %q, want c3 (the trigger comment)", got["thread-A"])
 		}
-		if got["thread-B"] != "thread-B" {
-			t.Errorf("other thread parent = %q, want its root thread-B", got["thread-B"])
+		// The other thread replies under the specific comment that mentioned the
+		// agent (a mid-thread reply), not the thread root — fixes the placement
+		// asymmetry from the first cut.
+		if got["thread-B"] != "c2" {
+			t.Errorf("other thread parent = %q, want c2 (the specific mentioning comment)", got["thread-B"])
 		}
 	})
 
@@ -752,6 +755,30 @@ func TestCommentReplyThreadsGrouping(t *testing.T) {
 			t.Fatalf("ordinary single-comment run must not fan out; got %+v", targets)
 		}
 	})
+
+	t.Run("non-trigger thread replies under its newest mention, not root", func(t *testing.T) {
+		// Two mid-thread mentions in thread-B (oldest c1, newer c2); the reply
+		// should target the newest specific comment (c2), not the root thread-B.
+		task := Task{
+			TriggerCommentID: "c9",
+			TriggerThreadID:  "thread-A",
+			CoalescedComments: []CoalescedCommentData{
+				{ID: "c1", ThreadID: "thread-B", Content: "older mention", CreatedAt: "2026-07-10T01:00:00Z"},
+				{ID: "c2", ThreadID: "thread-B", Content: "newer mention", CreatedAt: "2026-07-10T02:00:00Z"},
+			},
+		}
+		targets := commentReplyThreads(task)
+		got := map[string]string{}
+		for _, tgt := range targets {
+			got[tgt.ThreadID] = tgt.ParentID
+		}
+		if got["thread-B"] != "c2" {
+			t.Errorf("thread-B parent = %q, want newest mention c2 (not root)", got["thread-B"])
+		}
+		if got["thread-A"] != "c9" {
+			t.Errorf("trigger thread parent = %q, want trigger c9", got["thread-A"])
+		}
+	})
 }
 
 // TestBuildCommentPromptCrossThreadFansOutReplies is the end-to-end prompt