address review feedback for streamline-mirroring refactor

Required:

• explain.loadPrimaryMetadataRootTree: drop the Read != Primary clause from the origin-fallback gate. The function reads Primary's tree, so whether reads bootstrap from origin is irrelevant.
• strategy: inline getCommittedReadStore (one-line wrapper) into its sole caller in manual_commit_condensation.

Improvements:

• CommittedRefs.PrimaryFetchableFromOrigin: require Primary.IsBranch(). Non-branch refs don't get refs/remotes/origin/<name> shadows.
• CommittedRefs.ReadBootstrappableFromOrigin: extract the predicate used at three sites (resume bootstrap gate, resume remote-metadata check, GitStore fallback) so the gate has a name.
• attach, explain: drop the redundant outer HasMirror() guard around MirrorCommittedMetadataRef; the function already short-circuits.
• push_common: inline setLocalAfterSync into fetchAndRebaseSessionsCommon and resolve refs once at the top instead of per advance step.
• cleanup: pass already-resolved refs into the final AdvanceCommittedPrimary call instead of re-resolving.
• v1_custom_ref_mirror_test: populate Push in v1CustomRefsForTest so the helper matches what the resolver actually produces.
• push_common_test: add TestFetchAndRebase_NonBranchRef.
• attach.checkpointPresentLocally: route the read through a Primary-bound store so the result reflects the write target, not the configured read ref.
• push_common.pushRefIfNeeded: log a Debug when OpenRepository fails instead of silently returning nil.