fix(core): use unified release body from root CHANGELOG.md in fixed mode

Fixed mode writes a single root CHANGELOG.md, but buildFixedReleaseBody()
was iterating per-package and creating duplicate sections or falling back
to per-package commit logs. Simplify to delegate to buildReleaseBody()
without pkgPath, which naturally reads the root CHANGELOG and produces
a unified commit list as fallback.

Author: syi0808

packages/core/src/tasks/release-notes.ts

@@ -113,28 +113,19 @@ export async function buildFixedReleaseBody(
 ): Promise<string> {
   const { packages, tag, repositoryUrl } = options;
 
-  // Resolve previousTag once and pass it down to avoid redundant git calls per package
-  const git = new Git();
-  const previousTag = (await git.previousTag(tag)) || (await git.firstCommit());
-
-  const compareLink = `**Full Changelog**: ${repositoryUrl}/compare/${previousTag}...${tag}`;
-
-  const sections: string[] = [];
-  for (const pkg of packages) {
-    const body = await buildReleaseBody(ctx, {
-      pkgPath: pkg.pkgPath,
-      version: pkg.version,
-      tag,
-      repositoryUrl,
-      appendCompareLink: false,
-      previousTag,
-    });
-
-    sections.push(`## ${pkg.pkgName} v${pkg.version}\n\n${body}`);
-  }
-
-  const joined = sections.join("\n\n---\n\n");
-  return `${joined}\n\n${compareLink}`;
+  // In fixed mode all packages share the same version (enforced by FixedVersionPlan).
+  // Use the first entry; the type allows heterogeneous versions but fixed mode never produces them.
+  const version = packages[0]?.version ?? tag.replace(/^v/, "");
+
+  // Delegate to buildReleaseBody WITHOUT pkgPath for unified body.
+  // This reads root CHANGELOG.md (since pkgPath is undefined),
+  // and falls back to unified commit list — matching how fixed mode
+  // writes a single root CHANGELOG.md.
+  return buildReleaseBody(ctx, {
+    version,
+    tag,
+    repositoryUrl,
+  });
 }
 
 function extractChangelog(