Parse target HEAD symref into TargetFeatures
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Read symref=HEAD:<ref> from the receive-pack advertisement the same way the source's RefService already does, and store it on TargetFeatures. The session can then surface it without needing to keep the raw *packp.AdvRefs around at the top level.
Refs #45.
Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com Entire-Checkpoint: 9d1c1f5b25ea
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