fix(mcp): bold labels instead of ATX headings in tool results (#778) (#951)

MCP tool results used Markdown ATX headings (##/###/####) for section
headers — the status summary, each search hit, every file section in an
exploration — which Markdown-rendering clients (e.g. the Claude Code
VSCode extension) blow up to H1–H4 font size, filling the transcript with
oversized lines (worst on search/explore, where the noise scales with
result count). Swap them all for bold labels, which render at body size
while keeping the same structure. CLI/TTY output (ContextBuilder) is
unchanged — the issue notes it's fine.

The format is parse-coupled, so kept in sync:
- The explore truncation boundary and the offload chunker
  (reasoning/reasoner.ts) both key off the per-file header, now a unique
  `**`-prefixed marker emitted via a shared fileSectionHeader() helper.
- Updated the offload strip regexes and switched the opt-in report-style
  prompt off ATX headings (same client, same rendering issue).
- Updated test helpers (sectionFor, sourcedFiles, the callers
  section-boundary scan) that scanned the old markers.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

__tests__/mcp-staleness-banner.test.ts

@@ -175,7 +175,7 @@ describe('MCP staleness banner', () => {
 
     const res = await handler.execute('codegraph_status', {});
     const text = res.content[0].text;
-    expect(text).toContain('### Pending sync:');
+    expect(text).toContain('**Pending sync:');
     expect(text).toContain('src/charlie-only.ts');
     // Status embeds the info first-class, so the auto-banner is suppressed.
     expect(text.startsWith('⚠️')).toBe(false);
@@ -204,7 +204,7 @@ describe('MCP staleness banner', () => {
 
     const res = await handler.execute('codegraph_status', {});
     const text = res.content[0].text;
-    expect(text).toContain('### Auto-sync disabled:');
+    expect(text).toContain('**Auto-sync disabled:');
     expect(text).toContain('OS watch/file limit exhausted');
     // status renders the notice inline, so the auto-banner is not also prepended.
     expect(text.startsWith('⚠️')).toBe(false);