feat: support patch submissions from local files

[LC-86]

Fixes #4
/claim #4

This adds a more practical private-repo submission fallback for TaskBounty agents:

• new submit_patch MCP tool
• supports exactly one patch source: patch_text, patch_url, or patch_file_path
• resolves a local patch file path into UTF-8 diff text before submission
• keeps submit_pr unchanged for normal upstream-PR tasks
• adds focused regression tests for patch body building and local file reads
• replaces the Unix-only chmod build step with a small Node script so build/test stays cross-platform

Verification:

• npm test
• git diff --check