fix(engine): give merge commits a real subject in fallback paths

Three merger fallback commit paths (auto-resolve-all-conflicts,
-X theirs/ours side strategy, AI-agent-didn't-commit) hard-coded
`feat(FN-XXXX): merge fusion/fn-xxxx` as the subject and never used
the AI subject summarizer. Route them through buildDeterministicMergeMessage
so they pick up aiSubject when available.

When the AI subject summarizer returns null, derive the subject from
the branch's first step commit (with conventional-commit prefix
stripped, plus `(+N more)` for multi-commit branches) instead of the
bare `merge <branch>` template.

Bump DEFAULT_COMMIT_SUBJECT_TIMEOUT_MS 15s → 30s so slow-first-token
providers complete instead of silently falling back.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/merger.test.ts

@@ -1489,8 +1489,13 @@ describe("aiMergeTask — includeTaskIdInCommit setting", () => {
       (call) => String(call[0]).includes("git commit"),
     );
     expect(commitCall).toBeDefined();
-    expect(String(commitCall![0])).toContain("feat: merge");
+    // Subject must use bare `feat:` prefix (no task-id scope) when
+    // includeTaskIdInCommit=false. The summary portion is derived from the
+    // step commit log or AI subject, so we don't pin its exact text — just
+    // assert the prefix shape.
+    expect(String(commitCall![0])).toMatch(/git commit -m "feat: \S/);
     expect(String(commitCall![0])).not.toContain("feat(KB-050)");
+    expect(String(commitCall![0])).not.toContain("feat(FN-050)");
   });
 });
 

packages/engine/src/merger.ts

@@ -1109,10 +1109,39 @@ async function generateAiMergeSubject(
   }
 }
 
+/**
+ * Derive a non-AI subject summary from the branch's step commit log. The log
+ * is `- subj1\n- subj2\n…` (most recent first). We use the first subject with
+ * its conventional-commit prefix stripped (to avoid `feat: feat(...): …`),
+ * and tack on `(+N more)` when the branch has multiple step commits. This is
+ * the fallback used when `summarizeCommitSubject` returns null — it conveys
+ * what landed instead of the bare `merge <branch>` template.
+ */
+function deriveDeterministicSubjectSummary(commitLog: string): string | null {
+  const lines = commitLog
+    .split(/\r?\n/)
+    .map((l) => l.trim())
+    .filter((l) => l.length > 0);
+  if (lines.length === 0) return null;
+
+  const stripBullet = (l: string) => l.replace(/^[-*]\s+/, "").trim();
+  const stripConventional = (l: string) =>
+    l.replace(/^[a-z]+(?:\([^)]+\))?!?:\s*/i, "").trim();
+
+  const first = stripConventional(stripBullet(lines[0]));
+  if (!first) return null;
+
+  const extras = lines.length - 1;
+  const summary = extras > 0 ? `${first} (+${extras} more)` : first;
+  return summary;
+}
+
 /**
  * Build the canonical merge commit message from the branch's step commits.
- * Subject is `feat[(taskId)]: <aiSubject>` when the AI subject summarizer
- * produced one, else falls back to `feat[(taskId)]: merge <branch>`.
+ * Subject preference order:
+ *   1. AI summarizer (`summarizeCommitSubject`) when it succeeded
+ *   2. First step commit subject (with conventional prefix stripped) + `(+N more)`
+ *   3. `merge <branch>` (last-resort, only when no step commits exist)
  */
 async function buildDeterministicMergeMessage(params: {
   taskId: string;
@@ -1125,7 +1154,13 @@ async function buildDeterministicMergeMessage(params: {
 }): Promise<{ subjectArg: string; bodyArg: string }> {
   const { taskId, branch, commitLog, diffStat, includeTaskId, aiSummary, aiSubject } = params;
   const prefix = includeTaskId ? `feat(${taskId})` : "feat";
-  const subjectSummary = aiSubject?.trim().length ? aiSubject.trim() : `merge ${branch}`;
+  const trimmedAiSubject = aiSubject?.trim() ?? "";
+  const derived = trimmedAiSubject.length === 0
+    ? deriveDeterministicSubjectSummary(commitLog ?? "")
+    : null;
+  const subjectSummary = trimmedAiSubject.length > 0
+    ? trimmedAiSubject
+    : (derived ?? `merge ${branch}`);
   const subject = `${prefix}: ${subjectSummary}`;
 
   const trimmedCommitLog = commitLog?.trim() ?? "";
@@ -4258,12 +4293,19 @@ async function executeMergeAttempt(
               settings: settings as Settings,
               signal: options.signal,
             });
-            const escapedLog = safeBody.replace(/"/g, '\\"');
-            const fallbackPrefix = includeTaskId ? `feat(${taskId})` : "feat";
             const authorArg = getCommitAuthorArg(settings);
             const trailerArg = buildTaskIdTrailerArg(taskId);
+            const { subjectArg, bodyArg } = await buildDeterministicMergeMessage({
+              taskId,
+              branch,
+              commitLog,
+              diffStat,
+              includeTaskId,
+              aiSummary: safeBody,
+              aiSubject,
+            });
             await execAsync(
-              `git commit -m "${fallbackPrefix}: merge ${branch}" -m "${escapedLog}"${trailerArg}${authorArg}`,
+              `git commit ${subjectArg} ${bodyArg}${trailerArg}${authorArg}`,
               { cwd: rootDir },
             );
             mergerLog.log(`${taskId}: committed after auto-resolving all conflicts`);
@@ -4391,6 +4433,8 @@ async function executeMergeAttempt(
       branch,
       commitLog,
       diffStat,
+      aiSummary,
+      aiSubject,
       includeTaskId,
       hasConflicts,
       simplifiedContext: attemptNum === 2,
@@ -4521,7 +4565,7 @@ async function attemptWithSideStrategy(
   side: "theirs" | "ours" = "theirs",
   aiTracker?: AiInvocationTracker,
 ): Promise<boolean> {
-  const { rootDir, branch, commitLog, diffStat, includeTaskId, sourceIssueRef, taskId, store, settings, testCommand, buildCommand, testSource, buildSource } = params;
+  const { rootDir, branch, commitLog, diffStat, aiSummary, aiSubject, includeTaskId, sourceIssueRef, taskId, store, settings, testCommand, buildCommand, testSource, buildSource } = params;
 
   mergerLog.log(`${taskId}: attempting merge with -X ${side} strategy`);
 
@@ -4583,13 +4627,20 @@ async function attemptWithSideStrategy(
       settings: settings as Settings,
       signal: params.options.signal,
     });
-    const escapedLog = safeBody.replace(/"/g, '\\"');
-    const fallbackPrefix = includeTaskId ? `feat(${taskId})` : "feat";
     const authorArg = getCommitAuthorArg(settings);
     const trailerArg = buildTaskIdTrailerArg(taskId);
     const issueRefBodyArg = sourceIssueRef ? ` -m "Ref: ${sourceIssueRef}"` : "";
+    const { subjectArg, bodyArg } = await buildDeterministicMergeMessage({
+      taskId,
+      branch,
+      commitLog,
+      diffStat,
+      includeTaskId,
+      aiSummary: aiSummary?.trim().length ? aiSummary : safeBody,
+      aiSubject,
+    });
     await execAsync(
-      `git commit -m "${fallbackPrefix}: merge ${branch} (auto-resolved)" -m "${escapedLog}"${issueRefBodyArg}${trailerArg}${authorArg}`,
+      `git commit ${subjectArg} ${bodyArg}${issueRefBodyArg}${trailerArg}${authorArg}`,
       { cwd: rootDir },
     );
     mergerLog.log(`${taskId}: committed with -X ${side} auto-resolution`);
@@ -4626,6 +4677,8 @@ interface AiAgentParams {
   branch: string;
   commitLog: string;
   diffStat: string;
+  aiSummary?: string | null;
+  aiSubject?: string | null;
   includeTaskId: boolean;
   hasConflicts: boolean;
   simplifiedContext: boolean;
@@ -4668,6 +4721,8 @@ async function runAiAgentForCommit(params: AiAgentParams): Promise<{ success: bo
     branch,
     commitLog,
     diffStat,
+    aiSummary,
+    aiSubject,
     includeTaskId,
     hasConflicts,
     simplifiedContext,
@@ -4912,13 +4967,20 @@ async function runAiAgentForCommit(params: AiAgentParams): Promise<{ success: bo
           settings: settings as Settings,
           signal: options.signal,
         });
-        const escapedLog = safeBody.replace(/"/g, '\\"');
-        const fallbackPrefix = includeTaskId ? `feat(${taskId})` : "feat";
         const authorArg = getCommitAuthorArg(settings);
         const trailerArg = buildTaskIdTrailerArg(taskId);
         const issueRefBodyArg = sourceIssueRef ? ` -m "Ref: ${sourceIssueRef}"` : "";
+        const { subjectArg, bodyArg } = await buildDeterministicMergeMessage({
+          taskId,
+          branch,
+          commitLog,
+          diffStat,
+          includeTaskId,
+          aiSummary: aiSummary?.trim().length ? aiSummary : safeBody,
+          aiSubject,
+        });
         await execAsync(
-          `git commit -m "${fallbackPrefix}: merge ${branch}" -m "${escapedLog}"${issueRefBodyArg}${trailerArg}${authorArg}`,
+          `git commit ${subjectArg} ${bodyArg}${issueRefBodyArg}${trailerArg}${authorArg}`,
           { cwd: rootDir },
         );
       } else {