fix: skip rebase before first branch push

[eshulman2]

Summary

• treat a missing remote implementation branch as the normal first-push case
• preserve fetch failures while skipping only the invalid rebase
• add regression coverage for missing and existing remote branches

Testing

• UV_CACHE_DIR=/tmp/uv-cache uv run pytest -q tests/unit/workspace/test_git_ops_sync.py
• UV_CACHE_DIR=/tmp/uv-cache uv run ruff check src/forge/workspace/git_ops.py tests/unit/workspace/test_git_ops_sync.py