Fix doc/comment accuracy for pushed v1.1 ref (PR #1380 review)

Address Copilot review comments: with milestone 2 pushing v1.1 to the remote, three places still described it as a local-only, never-pushed mirror.

• committed_refs_test.go: the ReadBootstrappableFromOrigin case comment now attributes the false result to v1.1 being a non-branch ref (no origin-tracking shadow) with Read != Primary, not to being "local-only".
• sessions-and-checkpoints.md: PrePush "pushes" (not "advances") the refs in the push set.
• paths.go: MetadataRefName doc no longer says the ref is local-only / never pushed; it documents the PrePush push and the no-origin-shadow read behavior.