Implement core patch export system

Add functionality to create uncompressed, human-readable git patches
for export. This enables users to generate patches that can be manually
applied or shared, without the compression used for internal checkpoints.

Changes:
- Add ExportPatchResult struct with patch content, file count, and line stats
- Add create_export_patch() function that generates diffs against a base SHA
- Add get_head_sha() utility function
- Add parse_diff_stat() helper to extract line counts from git output
- Add CreateExportPatch command to daemon protocol
- Add ExportPatchCreated response message to protocol
- Add handler in task manager to process export patch requests
- Add server-side handling to broadcast patch results to UI

The export patch system automatically finds the merge-base when no base
SHA is provided, trying upstream tracking branch first, then common
default branches (origin/main, origin/master, main, master).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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