Pull Request Summary: Add SSH Transport Support

This PR implements SSH transport support in the git-sync project, allowing users to interact with repositories over SSH. The implementation includes a new transport mechanism and integrates error handling, testing, and documentation.

Commit Stack:

1. f44c0ccc gitproto: extract transport interface and rename HTTPConn
2. ebfc0d98 gitproto: add SSH transport via per-RPC ssh exec
3. 74a4cf49 syncer: warn that SSH stats and progress are incomplete
4. c0b12714 tests: add end-to-end SSH transport coverage
5. 98ca4e96 docs: document SSH support and caveats
6. 3cc55f83 lint: fix transport linter findings
7. 9c50a33f ssh: clean up failed info-refs startup

Key Features:

• SSH Transport Integration: Added a new transport mechanism for SSH connections that supports ssh://, SCP-style git@host:path.git, and git+ssh:// URLs.
• Error Handling: Ensures proper cleanup in case of failure, preventing leaked processes.
• Warning for Incomplete Stats: Introduced user-facing warnings for scenarios where byte-counted throughput is not available when using SSH.
• Testing: Comprehensive testing has been implemented, including unit tests for the SSH transport and end-to-end tests using a local shim.
• Documentation: Updates to the README and usage documentation to inform users about SSH support and its limitations.

Remaining Caveat:

• The SSH implementation currently does not support byte-counted throughput statistics when using --progress and --show-stats. This limitation is documented in the warning messages.

Action Items:

• Request review and merge.
• After merging, consider adding a follow-up PR for a more robust end-to-end test using Docker-based sshd.

For further details, please review the performance and coverage achieved by the end-to-end tests and unit tests included in this PR.