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__/offload.test.ts

@@ -231,16 +231,16 @@ describe('reasoning offload', () => {
   describe('stripAgentDirectives', () => {
     it('drops the agent-directed header but keeps source sections', () => {
       const ctx = [
-        '## Exploration: how does X work',
+        '**Exploration: how does X work**',
         'Found 12 symbols across 3 files.',
         '',
-        '#### src/a.ts — foo(function)',
+        '**`src/a.ts`** — foo(function)',
         'code body',
       ].join('\n');
       const stripped = stripAgentDirectives(ctx);
-      expect(stripped).not.toContain('## Exploration:');
+      expect(stripped).not.toContain('**Exploration:');
       expect(stripped).not.toContain('Found 12 symbols');
-      expect(stripped).toContain('#### src/a.ts');
+      expect(stripped).toContain('**`src/a.ts`');
       expect(stripped).toContain('code body');
     });
   });