Entire checkpoint branch format (tests/test_entire_import_0_19.py builds its
fixture branch in-test from this layout; no bytes are vendored).

Read from: https://github.com/entireio/cli
Commit:    54b6a5cac34e5ff939e6e5871168da9ffd4d187a (2026-10-30,
           describe: v0.12.3-nightly.202610310614.54b6a5cac)
Files:     cmd/entire/cli/paths/paths.go       (branch name, file names)
           api/checkpoint/metadata.go           (Metadata, CheckpointSummary,
                                                 SessionFilePaths, layout doc)
           cmd/entire/cli/checkpoint/id/id.go   (12-hex / 26-char ULID ids,
                                                 Path() = id[:2] + "/" + id[2:])
           cmd/entire/cli/agent/chunking.go     (full.jsonl.%03d chunks)
           cmd/entire/cli/agent/registry.go     (agent type strings)
           cmd/entire/cli/agent/types/token_usage.go

Layout on refs/heads/entire/checkpoints/v1:
  <id[:2]>/<id[2:]>/metadata.json      CheckpointSummary (sessions[].metadata =
                                       "/<id[:2]>/<id[2:]>/<N>/metadata.json")
  <id[:2]>/<id[2:]>/<N>/metadata.json  session Metadata (session_id, agent,
                                       model, files_touched, token_usage, ...)
  <id[:2]>/<id[2:]>/<N>/full.jsonl     raw agent transcript (+ .001 chunks;
                                       full.log in old versions)
  <id[:2]>/<id[2:]>/<N>/prompt.txt
