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>

scripts/agent-eval/offload-eval-metrics.mjs

@@ -48,7 +48,7 @@ for (const line of lines) {
       // "### Referenced source — verbatim" appendix). A refs call that cited nothing
       // valid falls back to RAW source, which is correctly counted as a raw explore below.
       if (/Synthesized by CodeGraph|### Referenced source — verbatim/.test(text)) { offloadAnswers.push(text); exploreResults++; }
-      else if (/Found \d+ symbols? across|## Exploration:/.test(text)) exploreResults++;
+      else if (/Found \d+ symbols? across|\*\*Exploration:/.test(text)) exploreResults++;
     }
   }
   if (ev.type === 'result') result = ev;