checkpoint: extract the persistent contract to api/checkpoint

Move the persistent-checkpoint contract — the persisted document types (Metadata, CheckpointSummary, Summary, Attribution, ...), the option types (WriteOptions/UpdateOptions/PrecomputedTranscriptBlobs), the reader/writer interfaces, the Write request union, the sentinel errors, and the CheckpointVersionBranchV1 const — into a new github.com/entireio/cli/api/checkpoint package, born with the persistent vocabulary.

The contract depends only on leaf packages (agent/types, checkpoint/id, redact, go-git plumbing), so a storage backend can implement it without the CLI's agent/TUI/git machinery. The git implementation (GitStore, Open, the Stores facade, AuthorReader, and the ephemeral shadow-branch surface) stays in cmd/entire/cli/checkpoint, which imports the contract and re-exports every moved symbol via aliases (aliases.go) — all existing call sites compile unchanged.

Notes:

• PrecomputedTranscriptBlobs.isUsable is now exported (IsUsable); its one caller is across the new package boundary.
• The Write union is sealed to api/checkpoint, so the impl-package unknown-request test was removed (no longer expressible); per-request dispatch coverage remains.
• normalizeCheckpointSummary stays in the impl (read-time normalization).