feat(sync): add SFTP pull direction

[risu729]

Closes #413.

Summary

• add biwa sync --pull for copying eligible remote-generated artifacts back into the local sync root
• make pull non-destructive by default: only existing generated local files are updated unless --overwrite is passed
• add --overwrite for explicit pull overwrites, local file deletion, remote-only file creation, and empty-directory mirroring
• apply ignore/include/exclude filters to remote pull candidates and keep rejecting remote symlink entries
• keep pull remote-state inspection read-only for existing remote directories while retaining push-side permission normalization
• update sync behavior docs and generated CLI usage

Testing

• LINT=true mise run check
• cargo test ssh::sync::tests
• cargo test --test ssh_e2e_sync e2e_sync_pull

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	biwa-docs	6997c61	Commit Preview URL Branch Preview URL	May 18 2026, 06:49 PM

[codecov]

Codecov Report

❌ Patch coverage is 64.20048% with 150 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.32%. Comparing base (7e848bd) to head (6997c61).
⚠️ Report is 278 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ssh/sync.rs	65.93%	139 Missing ⚠️
src/cli/sync.rs	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #634       +/-   ##
===========================================
+ Coverage   41.28%   51.32%   +10.04%     
===========================================
  Files          16       25        +9     
  Lines         671     2457     +1786     
===========================================
+ Hits          277     1261      +984     
- Misses        394     1196      +802     

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[gemini-code-assist]

Code Review

This pull request introduces a pull synchronization mode to biwa, enabling file transfers from a remote host to the local sync root. The changes include adding a --pull flag to the sync command, updating documentation, and implementing the core pull logic with safety guards against unsafe path traversal and symlink manipulation. Review feedback highlights opportunities to improve performance by using asynchronous file system operations and ensuring that local file and directory permissions remain consistent with the project's security policies (0700 for directories and 0600 for files). Additionally, there is a suggestion to refine how the tool handles local symlinks during the pull process to make the synchronization more robust.