fix(composer): past-tense plan and thought headlines in the activity pill

- Normalize plan items in getActivityHeadline (agentSessionTranscriptPresentation.ts):
  the fallthrough previously returned the raw item title, so ACP plan
  updates headlined the pill as a bare "Plan" / "Plan updated" next to the
  past-tense tool verbs ("Read foo.ts", "Ran …") — now "Created plan" for
  the initial plan and "Updated plan" for update markers (keyed off isUpdate)
- Normalize thought items the same way: "Thinking" → "Thought", and the
  plan-titled thought that rendered "Planning" → "Planned"
- Assistant-message headlines (first line of the streamed reply) are
  deliberately untouched — they are content, not action labels
- Add test coverage for all four normalized cases

Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

desktop/src/features/agents/ui/agentSessionTranscriptPresentation.test.mjs

@@ -94,6 +94,57 @@ test("getActivityHeadline basenames path-like tool objects", () => {
   );
 });
 
+test("getActivityHeadline renders plan and thought items in past tense", () => {
+  // Bare "Plan" / "Thinking" titles must not headline next to the past-tense
+  // tool verbs ("Read foo.ts", "Ran …").
+  assert.equal(
+    getActivityHeadline({
+      id: "plan:1",
+      type: "plan",
+      renderClass: "plan",
+      title: "Plan",
+      text: "1. Do the thing",
+      timestamp: baseTimestamp,
+    }),
+    "Created plan",
+  );
+  assert.equal(
+    getActivityHeadline({
+      id: "plan:1:update:x",
+      type: "plan",
+      renderClass: "plan",
+      title: "Plan updated",
+      text: "Now doing step 2",
+      timestamp: baseTimestamp,
+      isUpdate: true,
+      targetId: "plan:1",
+    }),
+    "Updated plan",
+  );
+  assert.equal(
+    getActivityHeadline({
+      id: "thought:1",
+      type: "thought",
+      renderClass: "thought",
+      title: "Thinking",
+      text: "hmm",
+      timestamp: baseTimestamp,
+    }),
+    "Thought",
+  );
+  assert.equal(
+    getActivityHeadline({
+      id: "thought:2",
+      type: "thought",
+      renderClass: "thought",
+      title: "Plan",
+      text: "plan-shaped thought",
+      timestamp: baseTimestamp,
+    }),
+    "Planned",
+  );
+});
+
 test("getActivityHeadline keeps shell commands whole (no basenaming)", () => {
   assert.equal(
     getActivityHeadline(

desktop/src/features/agents/ui/agentSessionTranscriptPresentation.ts

@@ -64,6 +64,11 @@ function terseActionObject(summary: CompactToolSummary): string | null {
  * reduced to their basename), e.g. "Read foo.ts" rather than
  * "Read file · src/agents/ui/foo.ts" — so the composer pill's 200px cap
  * shows the informative part instead of ellipsizing a long preview.
+ *
+ * Action headlines are past-tense verb phrases throughout ("Read foo.ts",
+ * "Thought", "Updated plan") — plan and thought items are normalized here
+ * so no bare noun ("Plan") or present participle ("Thinking") leaks into
+ * the pill next to the past-tense tool verbs.
  */
 export function getActivityHeadline(item: TranscriptItem): string | null {
   if (item.type === "tool") {
@@ -93,7 +98,14 @@ export function getActivityHeadline(item: TranscriptItem): string | null {
   }
 
   if (item.type === "thought") {
-    return item.title === "Plan" ? "Planning" : item.title;
+    if (item.title === "Plan") {
+      return "Planned";
+    }
+    return item.title === "Thinking" ? "Thought" : item.title;
+  }
+
+  if (item.type === "plan") {
+    return item.isUpdate ? "Updated plan" : "Created plan";
   }
 
   if (item.type === "metadata") {