Update docs: target HEAD detection landed, mismatch is observable

usage.md now describes both sourceHead and targetHead, the upload-pack round-trip behind the target side, and the three practical mitigations for a default-branch mismatch:

• match defaults at init time (preferred)
• set HEAD manually post-sync (host API or git symbolic-ref)
• script around the probe output to detect mismatches before sync

The receive-pack-omits-HEAD limitation note moves to a smaller side remark; we now work around it rather than calling it a follow-up.

Refs #45.