feat(markdown): index documentation graph links

[QingNagi]

Why this feature is needed

Agent and skill workflows often mix Markdown instructions, phase documents, script templates, and implementation scripts. Important logic is not always in code; it is frequently defined in SKILL.md, runbooks, checklists, tables, and workflow docs.

This change extends CodeGraph’s Markdown indexing so Markdown files can participate in the same graph-based lookup flow as source code.

Markdown files are now indexed structurally: headings become searchable section nodes, stable table rows such as API-AUTH become searchable nodes, and Markdown references to files, scripts, command templates, and file::symbol targets are extracted as graph edges.

It also adds reverse mapping from code back to Markdown. For example, a Python function that opens docs/setup.md#database-setup now creates a graph edge from that function to the Markdown heading node. This lets agents move both ways: from docs to implementation, and from scripts back to the documentation rules or templates they depend on.

Main benefits:

• Direct lookup of Markdown headings and table rows.
• Fewer grep and full-file reads when locating documentation rules.
• Faster discovery of scripts/functions referenced by Markdown.
• Easier reverse tracing from code to related Markdown files or sections.
• Better validation that documentation and implementation stay aligned.

Example graph relationships:

API-AUTH          -> src/auth.ts::loginUser
API-AUTH          -> src/auth.ts::refreshToken
initializeDatabase -> docs/setup.md#database-setup
SETUP_DOC         -> docs/setup.md

Validation covered Markdown extraction, file-symbol resolution, Markdown anchor resolution, and full-pipeline doc/code graph edges.

Infrastructure
src/types.ts — added markdown to the Language union
src/extraction/grammars.ts — added .md, .mdx, and .markdown extension mapping; marked Markdown as a supported custom extractor language
src/extraction/markdown-extractor.ts — new lightweight Markdown extractor for headings, table rows, links, command templates, and file::symbol references
src/extraction/tree-sitter.ts — added Markdown extractor dispatch; added code-string scanning for Markdown path references such as docs/setup.md#database-setup
src/resolution/name-matcher.ts — added resolution for Markdown anchors and file::symbol references into target files
src/resolution/index.ts — updated fast pre-filtering so path-like references with #anchor can still resolve correctly

Test updates
__tests__/extraction.test.ts — added Markdown language detection, heading/link extraction, table row extraction, file-symbol extraction, and code-to-Markdown reference tests
__tests__/resolution.test.ts — added Markdown file, anchor, and file::symbol resolution tests
__tests__/integration/full-pipeline.test.ts — added full-pipeline tests for Markdown-to-script/function edges and code-to-Markdown heading edges