docs: add custom grammar and markdown special support sections to smart-explore/SKILL.md

- Add Custom Grammars (.claude-mem.json) section explaining how to register
  additional tree-sitter parsers for unsupported file extensions
- Add Markdown Special Support section documenting heading-based outline,
  code-fence search, section unfold, and frontmatter extraction behaviors
- Expand bundled language test to cover all 10 documented languages plus
  the plain-text fallback sentence to prevent partial doc regressions

Co-Authored-By: Claude <noreply@anthropic.com>

tests/utils/skill-docs-placement.test.ts

@@ -23,12 +23,24 @@ describe('skill docs placement (#1651)', () => {
   it('smart-explore/SKILL.md lists bundled languages', () => {
     const content = readFileSync(join(SKILLS_DIR, 'smart-explore/SKILL.md'), 'utf-8');
 
-    // Core languages must be documented
-    expect(content).toContain('JavaScript');
-    expect(content).toContain('TypeScript');
-    expect(content).toContain('Python');
-    expect(content).toContain('Go');
-    expect(content).toContain('Rust');
+    const expectedLanguages = [
+      'JavaScript',
+      'TypeScript',
+      'TSX / JSX',
+      'Python',
+      'Go',
+      'Rust',
+      'Ruby',
+      'Java',
+      'C',
+      'C++',
+    ];
+
+    for (const language of expectedLanguages) {
+      expect(content).toContain(language);
+    }
+
+    expect(content).toContain('Files with unrecognized extensions are parsed as plain text');
   });
 
   it('mem-search/SKILL.md does NOT contain tree-sitter or language grammar docs', () => {