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-corroboration-ranking.test.ts

@@ -28,11 +28,12 @@ import * as os from 'os';
 import CodeGraph from '../src/index';
 import { ToolHandler } from '../src/mcp/tools';
 
-/** Paths that explore rendered as full-body `#### <path> —` source sections. */
+/** Paths that explore rendered as full-body ``**`<path>`** —`` source sections.
+ *  Headers are bold labels, not ATX headings (issue #778). */
 function sourcedFiles(text: string): string[] {
   const out: string[] = [];
   for (const line of text.split('\n')) {
-    const m = line.match(/^#### (.+?) —/);
+    const m = line.match(/^\*\*`(.+?)`\*\* —/);
     if (m) out.push(m[1].trim());
   }
   return out;