Push checkpoints v1.1 ref to remote (rollout milestone 2)

Stacked on #1376 (feat/checkpoints-v1.1-streamline-mirroring), which made the CommittedRefs resolver the single source of the committed-metadata ref topology. This is the next rollout phase: get refs/entire/checkpoints/v1.1 onto the configured remote before a later milestone flips it to the primary write target.

• Resolver: append v1.1 to CommittedRefs.Push in opted-in mode, so PrePush pushes it alongside v1. Primary stays v1, so PrimaryFetchableFromOrigin (gated on Primary.IsBranch) and ReadBootstrappableFromOrigin (Read==Primary) are unchanged -- reads still treat v1.1 as local-only.
• PrePush: re-point the mirror at the primary tip before the push loop, so a tip left stale by an earlier best-effort advance is published correctly. Best-effort: a refresh failure is logged and never blocks the user's push.
• Tests: updated resolver Push expectations, added ReadBootstrappableFromOrigin coverage, inverted the milestone-1 "v1.1 never pushed" guard to assert v1.1 is pushed, and added an integration test that v1.1 lands on the remote at the v1 tip.
• Docs: CLAUDE.md and sessions-and-checkpoints.md no longer describe v1.1 as local-only.