Run e2e against a configurable checkpoint backend + git-refs CI
Commit

- settings: LoadCheckpointsConfig honors ENTIRE_CHECKPOINTS_PRIMARY (+ comma- separated ENTIRE_CHECKPOINTS_MIRRORS), env-wins-over-file, matching the other ENTIRE_* overrides. Backend selection only.
- e2e: make the harness backend-aware (e2e/testutil/backend.go: checkpointStoreMode, CheckpointState advance digest, checkpointBlobSpec/checkpointRefName/checkpointShard). Advance detection, metadata reads, CheckpointIDs, push, and artifact capture route through it; the v1-branch-specific alternate-object sync test skips under git-refs. AssertCheckpointIDFormat keeps the production Validate (hex or ULID).
- e2e TestMain maps E2E_CHECKPOINT_STORE (git-branch default, or git-refs) to the ENTIRE_CHECKPOINTS_PRIMARY override so every spawned binary/hook uses it.
- CI: e2e-checkpoint-store workflow with a checkpoint_store input; e2e/README doc.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: d9646529ed6e
Checkpoints
Checkpoint 1