checkpoint: split-store foundation (Open factory + Stores facade + reader split)

[Soph]

https://entire.io/gh/entireio/cli/trails/622

What

Foundation slice of the pluggable checkpoint-store work (#1433). This is the durable base carved out of #1474 — the first 7 commits, unchanged, all authored by @pfleidi:

• checkpoint.Open(ctx, repo, OpenOptions) factory — single construction site for the store, replacing ~15 scattered NewGitStore calls. Resolves the committed-ref topology and wires the on-demand blob fetcher in one place.
• checkpoint.Stores facade — Primary (committed store) + Temporary() (git-only shadow-branch capability) + resolved Refs(). attach injects its own instance via OpenOptions{Refs: …} to preserve PrimaryAsRead() pinning.
• Split reader interfaces + caller migration so call sites no longer type-assert back to *GitStore.
• getTemporaryStore strategy helper, reader edge-case fixes, attribution-reader narrowing, and the empty-summary lookup fix.

What this intentionally does not include

The committed_domain.go adapter tail from #1474 (the functional-options WriteSession/UpdateSession/UpdateCheckpoint writer surface) is dropped here on purpose. That writer shape is being reconsidered: per the design discussion on #1433, the target is a single unqualified Store.Write(ctx, WriteRequest) command union rather than a bag of functional options spread across session/checkpoint writers. Landing the options surface here and then replacing it would mean reviewing the writer API twice.

Stacked plan

This is PR 1 of 3, each one kind of change:

1. (this PR) durable foundation — Open factory, Stores facade, reader split, caller migration.
2. next, stacked — the Store.Write(ctx, WriteRequest) union (WriteSession/BackfillTranscript/BackfillSummary/BackfillAttribution) + the SessionRef-based reader domain. Pure interface+impl redesign, no file moves.
3. stacked on 2 — detangle agent/session/review/TUI deps from the contract (push TokenUsage/SkillEvent into a leaf types package) and relocate the contract to github.com/entireio/cli/api/checkpoint so the backend can import it.

Supersedes #1474.

Verification

go build ./..., go vet, mise run fmt (clean), mise run lint (0 issues), and unit tests for the touched packages (checkpoint/..., cmd/entire/cli, strategy) all green.

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Note

Medium Risk
Touches core checkpoint read/write and manual-commit save/rewind paths across many call sites, but the change is largely interface/factory wiring with added unit tests rather than new storage semantics.

Overview
Introduces checkpoint.Open as the single construction path for checkpoint storage, returning a Stores facade with Primary (CommittedStore) and Temporary() (TemporaryStore), plus resolved ref topology (attach still injects OpenOptions.Refs for read pinning).

Replaces the monolithic Store interface with narrower committed/temporary capability interfaces (CommittedStore, TemporaryStore, AuthorReader, etc.) and updates CLI/strategy call sites to depend on those types instead of *GitStore.

Manual-commit shadow-branch work (SaveStep, rewind, explain temp paths) now goes through getTemporaryStore; committed reads/writes stay on getCheckpointStore. Attribution and resume narrow further via local reader interfaces and shared read helpers; LookupSessionLog routes through ReadRawSessionLogForCheckpoint, with a fix so empty session summaries surface ErrCheckpointNotFound.

^{Reviewed by Cursor Bugbot for commit f39e8db. Configure here.}

[Copilot]

Pull request overview

This PR lays the groundwork for pluggable checkpoint storage by centralizing checkpoint-store construction and splitting the previously “fat” store surface into narrower committed vs temporary (shadow-branch) reader capabilities, then migrating key callers to the new abstractions.

Changes:

• Introduces checkpoint.Open(ctx, repo, OpenOptions) and a checkpoint.Stores facade to centralize store wiring (refs topology + blob fetcher) and expose Primary (committed) + Temporary() (git-only).
• Splits committed/temporary reader interfaces and migrates strategy + CLI call sites away from *checkpoint.GitStore where possible.
• Adds/updates tests to validate the narrowed interfaces and edge-case behavior (including empty-summary not-found handling).

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cmd/entire/cli/strategy/manual_commit.go	Adds getCheckpointStores helper and splits strategy access into committed vs temporary store capability.
cmd/entire/cli/strategy/manual_commit_test.go	Updates a condensation test to construct a store directly after getCheckpointStore now returns an interface.
cmd/entire/cli/strategy/manual_commit_rewind.go	Switches rewind-point discovery to use checkpoint.TemporaryStore.
cmd/entire/cli/strategy/manual_commit_git.go	Routes shadow-branch writes through getTemporaryStore (TemporaryStore).
cmd/entire/cli/strategy/manual_commit_condensation.go	Updates documentation/comments to reflect committed-store abstraction.
cmd/entire/cli/resume.go	Narrows resume’s dependency from *GitStore to a smaller reader interface.
cmd/entire/cli/resume_test.go	Adds a stub-based test proving resolveLatestCheckpoint only needs the narrowed reader surface.
cmd/entire/cli/explain.go	Moves author lookup behind optional checkpoint.AuthorReader and uses TemporaryStore for temporary explains.
cmd/entire/cli/checkpoint/store.go	Updates compile-time interface assertions to the new split interfaces.
cmd/entire/cli/checkpoint/open.go	Implements the Stores facade (Primary + Temporary() + resolved Refs()).
cmd/entire/cli/checkpoint/committed.go	Switches LookupSessionLog to committed-reader helpers instead of GitStore methods.
cmd/entire/cli/checkpoint/committed_reader_resolve.go	Defines new committed-store interfaces and adds reader helper utilities.
cmd/entire/cli/checkpoint/committed_reader_resolve_test.go	Adds coverage for the empty-summary not-found behavior in the new helpers.
cmd/entire/cli/checkpoint/checkpoint.go	Replaces the old combined Store interface with the git-only TemporaryStore surface.
cmd/entire/cli/attribution.go	Introduces a narrowed reader interface for attribution and normalizes nil summary handling.
cmd/entire/cli/attribution_test.go	Adds a stub-based test validating attribution’s narrower checkpoint reader dependency.
cmd/entire/cli/attach.go	Adjusts attach store opening to return CommittedStore capability (interface) instead of *GitStore.