Surface source HEAD symref in Result and ProbeResult

The session already captures source HEAD's symref target during ref discovery (it's used by bootstrap-batch trunk ordering), but never escapes to callers. Expose it on syncer.Result.SourceHEAD, syncer.ProbeResult.SourceHEAD, and the matching internalbridge fields, so library callers and the CLI can compare against the target's intended default branch without out-of-band metadata.

Human Lines() output picks up a "source-head: <ref>" line; JSON output gets execution.sourceHead for sync paths and sourceHead on probe. Field is omitempty so detached-HEAD sources omit it.

Refs #45.