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-output-budget.test.ts

@@ -206,8 +206,8 @@ describe('codegraph_explore output respects the adaptive budget', () => {
     const text = result.content?.[0]?.text ?? '';
     // Either there are relationships, or no edges were significant — both are fine.
     // We just want to confirm we did not accidentally gate it off.
-    const hasRelationships = text.includes('### Relationships');
-    const sourceFollowsHeader = text.indexOf('### Source Code') > 0;
+    const hasRelationships = text.includes('**Relationships');
+    const sourceFollowsHeader = text.indexOf('**Source Code') > 0;
     expect(hasRelationships || sourceFollowsHeader).toBe(true);
   });