feat(hex-ssh): Add recursive directory transfer workflow

[levnikolaevich]

Context

Current ssh-upload and ssh-download are strong single-file tools. Competitors with broader SFTP surfaces already cover directory-oriented workflows and recursive operations.

There is now a clear distinction between single-file transfer robustness and complete remote file sync workflow breadth. The next capability step is directory transfer, not more hidden complexity inside the current single-file tools.

This should follow #38, because recursive behavior should be built on top of proven real-server transfer semantics.

Why after #38

• directory transfer multiplies edge cases already present in single-file transfer
• backend differences, failure cleanup, and symlink handling should be informed by real-server evidence first
• separate tools are safer than silently extending current single-file behavior

What to do

• Add separate recursive directory transfer tools for SSH/SFTP workflows
• Support:
  • recursive upload of local directory trees
  • recursive download of remote directory trees
• Define explicit policy for:
  • overwrite behavior
  • symlinks
  • partial failure cleanup
  • empty directories
  • mixed existing trees
• Keep these as separate tools instead of hiding recursion behind the current single-file API
• Use a conservative default symlink policy and document it explicitly

Schema/API changes

• Add new directory-oriented tool names rather than extending ssh-upload / ssh-download implicitly
• Reuse the existing connection/security model where possible

Definition of Done

• Recursive upload tool exists
• Recursive download tool exists
• Overwrite behavior is explicit and documented
• Symlink policy is defined and tested
• Partial failure behavior is defined and tested
• Docs clearly separate single-file and directory workflows

Related issues

• Depends on: #38
• Complementary with: #37
• Should land before any advanced transfer modes from #41

Priority

Medium-high. This is the clearest functional breadth gap versus stronger SFTP competitors.