document v1.1 mirror as best-effort and the resume-promote exception

Earlier wording said the mirror is advanced "after active v1 write/fetch paths" without specifying who advances it, that failures are non-fatal, or that the resume bootstrap is excluded. Update CLAUDE.md and sessions-and-checkpoints.md to describe the actual contract:

• mirror is best-effort, gated on the checkpoints_version setting
• entire-managed v1 writes and fetches advance it; failures log but never fail the primary operation
• promoteRemoteTrackingMetadataBranch deliberately does not mirror and is skipped entirely when v1.1 is opted in
• read paths use the configured ref as-is