feat(merger,dashboard): use title-summarization model for commit body AI + reword settings

Wires the AI commit-body generator (introduced in 52928ff5e) to the
existing dedicated title-summarization model lane, and updates the
settings UI so users understand the model is now used for two
short-summary jobs instead of just one. Also de-duplicates the
project-scope settings UI which was exposing the same model setting in
two places.

Engine (merger.ts):
- aiGenerateCommitBody now prefers settings.titleSummarizerProvider /
  titleSummarizerModelId over the merger's default model. The
  summarization lane is the right tier for this work — small, fast,
  cheap. Falls back to the default merger model when the summarization
  lane is unset.

Dashboard (SettingsModal.tsx):
- MODEL_LANES summarization lane label updated:
  "Title Summarization Model" → "Title and Git Commit Message
  Summarization Model". Helper text updated to mention the dual purpose
  (auto-generated task titles + fallback merge commit message bodies).
  This change flows through automatically to BOTH the global model
  lanes view and any project-scope rendering of MODEL_LANES.
- Removed the duplicate summarization picker from the project-scope
  Model Lanes section: previously it appeared once under Model Lanes
  AND once under "AI Summarization". Now lives only in the dedicated
  picker so users have a single source of truth in project scope.
- The dedicated picker section heading + description rewritten:
  "AI Summarization" → "AI Title and Git Commit Message Summarization",
  with a paragraph explaining both jobs the model performs.
- Inner dropdown label updated for consistency.

Tests + checks: engine 2887/2887 pass, dashboard typecheck clean,
workspace lint clean.

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

packages/engine/src/merger.ts

@@ -1950,17 +1950,30 @@ async function aiGenerateCommitBody(opts: {
 
   let session: Awaited<ReturnType<typeof createResolvedAgentSession>>["session"] | undefined;
   try {
+    // Prefer the dedicated title-summarization model from settings — it's a
+    // small, fast model intended exactly for short summarization tasks like
+    // this. Falls back to the merger's default model only when the
+    // summarization model isn't configured.
+    const useTitleSummarizer =
+      !!opts.settings.titleSummarizerProvider && !!opts.settings.titleSummarizerModelId;
+    const provider = useTitleSummarizer
+      ? opts.settings.titleSummarizerProvider!
+      : (opts.settings.defaultProviderOverride && opts.settings.defaultModelIdOverride
+          ? opts.settings.defaultProviderOverride
+          : opts.settings.defaultProvider);
+    const modelId = useTitleSummarizer
+      ? opts.settings.titleSummarizerModelId!
+      : (opts.settings.defaultProviderOverride && opts.settings.defaultModelIdOverride
+          ? opts.settings.defaultModelIdOverride
+          : opts.settings.defaultModelId);
+
     const created = await createResolvedAgentSession({
       sessionPurpose: "merger",
       cwd: opts.rootDir,
       systemPrompt,
       tools: "readonly",
-      defaultProvider: opts.settings.defaultProviderOverride && opts.settings.defaultModelIdOverride
-        ? opts.settings.defaultProviderOverride
-        : opts.settings.defaultProvider,
-      defaultModelId: opts.settings.defaultProviderOverride && opts.settings.defaultModelIdOverride
-        ? opts.settings.defaultModelIdOverride
-        : opts.settings.defaultModelId,
+      defaultProvider: provider,
+      defaultModelId: modelId,
     });
     session = created.session;
     await session.prompt(userPrompt);