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__/same-name-disambiguation.test.ts

@@ -99,7 +99,10 @@ describe('same-named symbols across apps (#764)', () => {
     expect(out).toContain('apps/billing/src/users/user.service.ts');
     // …and the billing section must list the billing controller, not admin's.
     const billingSection = out.slice(out.indexOf('apps/billing/src/users/user.service.ts'));
-    const billingBody = billingSection.slice(0, billingSection.indexOf('###', 3) > 0 ? billingSection.indexOf('###', 3) : undefined);
+    // The next definition heading is a line-start bold label (issue #778: ATX `###`
+    // headings became `**…**`); billingSection starts mid-heading, so `\n**` finds it.
+    const nextDef = billingSection.indexOf('\n**');
+    const billingBody = billingSection.slice(0, nextDef > 0 ? nextDef : undefined);
     expect(billingBody).toContain('apps/billing/src/users/user.controller.ts');
     expect(billingBody).not.toContain('apps/admin/src/users/user.controller.ts');
   });