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__/explore-blast-radius.test.ts

@@ -55,7 +55,7 @@ describe('codegraph_explore — blast radius', () => {
     const res = await handler.execute('codegraph_explore', { query: 'target' });
     const text = res.content[0].text;
 
-    expect(text).toContain('### Blast radius');
+    expect(text).toContain('**Blast radius');
     expect(text).toContain('`target`');
     expect(text).toMatch(/caller/); // a caller count is reported
     // It names WHERE (the caller file) — not the caller's source body.