feat(mcp): support MCP Resources and Resource Templates

[allenporter]

Feature Description

Following the upgrade to MCP SDK v2.0.0, the Supernote MCP server should expose notebook contents as MCP Resources and Resource Templates using custom supernote:// URIs.

Currently, the MCP server only exposes Tools (search_notebook_chunks and get_notebook_transcript). Exposing native MCP Resources will allow AI clients (e.g. Claude Desktop, Cursor, Gemini) to directly attach, browse, and reference notebooks and pages into context.

Proposed Resource URI Scheme

• supernote://notebooks – List all available notebooks.
• supernote://notebooks/{file_id} – Notebook metadata and page index listing.
• supernote://notebooks/{file_id}/pages/{page_index}/transcript – Raw transcript text of a specific page.
• supernote://notebooks/{file_id}/pages/{page_index}/image – Rendered SVG/PNG vector image of handwritten note page.

Proposed Implementation

• Implement @mcp.resource() and @mcp.resource_template() endpoints in supernote/server/mcp/server.py.
• Add test coverage for resource listing and reading in tests/server/mcp/test_resources.py.
• Update MCP documentation in docs/mcp.md.