Docs: HEAD / default branch section

Adds a "HEAD / Default Branch" section to usage.md covering:

• where the source HEAD shows up (JSON and human output)
• bootstrap's first-pushed-wins ordering (the new behavior) and which hosts benefit from it (GitHub, GitLab on fresh repos)
• the wire-protocol limit ("no command for updating a remote symref") and the manual workarounds for hosts that don't infer from first push: match the default at init time, or set HEAD post-sync via the host API or git symbolic-ref

JSON Output section adds sourceHead to the documented keys.

Refs #45.