Minimize code comments in milestone-2 diff

Trim the comments added by this branch to the load-bearing "why", dropping restatement and obvious narration. No behavior change.

• committed_refs.go: drop the PrimaryFetchableFromOrigin invariant note (the test row covers it); keep why v1.1 is pushed.
• manual_commit_push.go: remove the self-evident call-site comment; shorten the refreshMirrorBeforePush docstring.
• committed_refs_test.go: tighten the two case comments (keep the non-branch / Read != Primary reason).
• remote_operations_test.go: shorten the docstring and tip-check comment; drop the comment narrating the v1-branch assertion.
• paths.go: shorten the MetadataRefName doc back toward its original length.
• push_common_test.go, v1_custom_ref_mirror_test.go: shorten test docstrings.