feat(mcp): add file output for large conversions

[dexhunter]

Summary

Add an opt-in output_file mode to the MarkItDown MCP tool for conversions that exceed an MCP client's response limit.

The default convert_to_markdown(uri) behavior is unchanged. When output_file is set, the tool writes the complete markdown to a new permission-restricted file and returns its absolute path instead of returning the full document through MCP.

Motivation

Issue #1332 reports a 51,164-token conversion exceeding a 25,000-token MCP response limit and requests an option to write the result to a file. Returning a short local path avoids the response-size failure while preserving the full converted document. Automatic pagination from #1333 remains separate work.

Safety

• Output files are created with mode 0600 on POSIX systems (platform-default permissions on Windows), and existing files are never overwritten.
• Partial files are removed when a write fails.
• The documentation covers shared-filesystem and Docker volume usage and makes clear that the caller owns cleanup.

Evidence

A strict synthetic large-document evaluation measured the MCP tool result:

baseline_response_bytes: 665020
output_file_response_bytes: 29
reduction: 99.996%
behavior_hash: 20dd505e39710c8c (unchanged)

Supplementary autoresearch: public dashboard.

Validation

hatch test
# 6 passed

uvx ruff check --fix .
uvx black --check .
git diff --check

Fixes #1332