Plumb SourceHEAD through bridge + render in human output

internalbridge.ProbeResult adds a SourceHEAD field; bridge.SyncResult's ExecutionSummary gets one too. CLI JSON output picks them up automatically through the From* conversions.

Result.Lines() and ProbeResult.Lines() emit a "source-head: <ref>" line when non-empty, matching the existing "source: <url>" / "ref-prefixes" style. The line gives users a quick way to see the source's default branch alongside the other discovery metadata.

Refs #45.