refactor(core,merger): consolidate AI commit-body summarization into ai-summarize.ts

Moves the commit-body AI helper out of merger.ts into the existing
core/ai-summarize.ts module so all short-summary AI work (titles,
chat titles, fallback merge commit bodies) shares one home with
consistent dispatch semantics, error handling, and session lifecycle.

Core (ai-summarize.ts):
- New `summarizeCommitBody(diffStat, rootDir, provider, modelId, opts)`
  exported alongside `summarizeTitle`. Same shape (provider/modelId
  args), same get-engine-or-bail dynamic loading via `getFnAgent`,
  same readonly-tools session, same disposal-in-finally pattern.
- Differs from `summarizeTitle` in three deliberate ways suited to the
  commit-body job:
    1. Returns null on any failure instead of throwing — the caller is
       always the merger, which has a deterministic fallback chain
       behind it. Throwing would force the merger to wrap every call
       in try/catch.
    2. Accepts an optional `signal` to forward engine-pause / shutdown
       cancellation, plus a configurable `timeoutMs` (default 30s)
       so a wedged AI session can't stall a merge indefinitely.
    3. Larger output ceiling (2000 chars vs title's 60) and larger
       input ceiling (4000 chars truncated diff) — commit bodies are
       multi-line and need more room than a 60-char title.
- Exported alongside `summarizeTitle` from `@fusion/core`. Constants
  (`COMMIT_BODY_SYSTEM_PROMPT`, `MAX_COMMIT_BODY_INPUT_LENGTH`,
  `MAX_COMMIT_BODY_LENGTH`, `DEFAULT_COMMIT_BODY_TIMEOUT_MS`) re-exported
  for callers that want to override behavior.

Engine (merger.ts):
- Dropped the local `aiGenerateCommitBody` function (~70 lines) — it
  duplicated the session-creation pattern from `summarizeTitle` while
  living in a place where future maintainers wouldn't think to look.
- `resolveSafeCommitBody` now imports `summarizeCommitBody` from
  `@fusion/core` and delegates. The cascade behavior is unchanged
  (commitLog → AI → diff stat → synthetic) and the title-summarizer
  model preference is preserved (provider/modelId resolved here, then
  passed through).

Tests:
- 6 new test cases in `ai-summarize.test.ts` covering:
  empty input → null, missing engine → null (graceful, never throws),
  missing engine + model selection → null, pre-aborted signal → null,
  custom timeout (returns quickly under 1s ceiling), exposed constants.
- Core: 3136/3136 pass (was 3130 — +6 new). Engine: 2887/2887 pass.
- Typecheck clean, workspace lint clean.

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

packages/core/src/__tests__/ai-summarize.test.ts

@@ -1,13 +1,18 @@
 import { describe, it, expect, beforeEach, vi } from "vitest";
 import {
   summarizeTitle,
+  summarizeCommitBody,
   checkRateLimit,
   getRateLimitResetTime,
   validateDescription,
   SUMMARIZE_SYSTEM_PROMPT,
+  COMMIT_BODY_SYSTEM_PROMPT,
   MAX_DESCRIPTION_LENGTH,
   MIN_DESCRIPTION_LENGTH,
   MAX_TITLE_LENGTH,
+  MAX_COMMIT_BODY_INPUT_LENGTH,
+  MAX_COMMIT_BODY_LENGTH,
+  DEFAULT_COMMIT_BODY_TIMEOUT_MS,
   MAX_REQUESTS_PER_HOUR,
   ValidationError,
   RateLimitError,
@@ -162,6 +167,73 @@ describe("ai-summarize", () => {
     });
   });
 
+  describe("summarizeCommitBody", () => {
+    it("returns null for empty diff stat (nothing to summarize)", async () => {
+      expect(await summarizeCommitBody("", "/tmp")).toBeNull();
+      expect(await summarizeCommitBody("   \n  ", "/tmp")).toBeNull();
+    });
+
+    it("returns null when AI engine is unavailable (graceful, never throws)", async () => {
+      const result = await summarizeCommitBody(
+        "src/foo.ts | 5 +++--\n1 file changed",
+        "/tmp",
+      );
+      expect(result).toBeNull();
+    });
+
+    it("returns null when AI engine is unavailable even with model selection", async () => {
+      // Should NOT throw — the contract is fail-soft so the merger can fall
+      // back to its deterministic body cascade without losing the merge.
+      const result = await summarizeCommitBody(
+        "src/foo.ts | 5 +++--\n1 file changed",
+        "/tmp",
+        "anthropic",
+        "claude-sonnet-4-5",
+      );
+      expect(result).toBeNull();
+    });
+
+    it("returns null when caller's abort signal is already aborted", async () => {
+      const ac = new AbortController();
+      ac.abort();
+      const result = await summarizeCommitBody(
+        "src/foo.ts | 5 +++--\n1 file changed",
+        "/tmp",
+        undefined,
+        undefined,
+        { signal: ac.signal },
+      );
+      expect(result).toBeNull();
+    });
+
+    it("respects custom timeout (returns null on timeout, never hangs)", async () => {
+      // Timeout is 1ms — faster than any real AI call, and the helper aborts
+      // the in-flight session and returns null instead of hanging the merge.
+      const start = Date.now();
+      const result = await summarizeCommitBody(
+        "src/foo.ts | 5 +++--\n1 file changed",
+        "/tmp",
+        undefined,
+        undefined,
+        { timeoutMs: 1 },
+      );
+      const elapsed = Date.now() - start;
+      expect(result).toBeNull();
+      // Belt-and-braces: even if the engine isn't available the call should
+      // return very quickly. The 1s ceiling guards against future regressions
+      // that might accidentally block.
+      expect(elapsed).toBeLessThan(1000);
+    });
+
+    it("exposes sensible constants", () => {
+      expect(COMMIT_BODY_SYSTEM_PROMPT.length).toBeGreaterThan(0);
+      expect(COMMIT_BODY_SYSTEM_PROMPT).toContain("commit message");
+      expect(MAX_COMMIT_BODY_INPUT_LENGTH).toBeGreaterThan(1000);
+      expect(MAX_COMMIT_BODY_LENGTH).toBeGreaterThan(100);
+      expect(DEFAULT_COMMIT_BODY_TIMEOUT_MS).toBeGreaterThan(0);
+    });
+  });
+
   // ── Error Classes ───────────────────────────────────────────────────────────
 
   describe("error classes", () => {