Document the HEAD propagation limitation and the workarounds

usage.md gains a "HEAD / Default Branch" section explaining:

• where to find the source's HEAD in JSON/human output
• why the target's HEAD isn't reconciled (no portable receive-pack symref-update; go-git alpha doesn't ship the capability anyway)
• two mitigations: match the default at init time (preferred), or set HEAD manually post-sync via git symbolic-ref or the host's API

The JSON Output section lists the new execution.sourceHead / sourceHead fields.

Refs #45.