feat(docs): add get_doc_as_markdown tool with comment context

Adds a new `get_doc_as_markdown` tool that converts Google Docs to clean
Markdown preserving formatting (headings, bold/italic/strikethrough, links,
code spans, ordered/unordered lists with nesting, and tables).

Optionally overlays comments with their anchor text (quotedFileContent) —
the specific text each comment is attached to — in two modes:
- inline: footnote-style references placed at the anchor text location
- appendix: all comments grouped at the bottom with blockquoted anchors

This gives AI agents full document context in a single tool call, unlike
get_doc_content which strips all formatting to plain text.

New files:
- gdocs/docs_markdown.py: Converter + comment formatting logic
- tests/gdocs/test_docs_markdown.py: 18 tests

Tool tier: extended (alongside search_docs, export_doc_to_pdf, etc.)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/tool_tiers.yaml

@@ -66,6 +66,7 @@ docs:
     - list_docs_in_folder
     - insert_doc_elements
     - update_paragraph_style
+    - get_doc_as_markdown
   complete:
     - insert_doc_image
     - update_doc_headers_footers