refactor(core): remove unused snapshot operations

[kitlangton]

What

Remove the confirmed-dead Snapshot.preview -> Git.tree.preview and Snapshot.checkout -> Git.tree.checkout chains while preserving snapshot capture, file listing, diffing, selective restore, path containment, and session revert behavior.

Preview history

Snapshot.preview and its synthetic-index Git.tree.preview implementation were introduced with the V2 snapshot system in #33226. They never gained a production caller; only the direct Git and Snapshot tests exercised them. This removes that test-only API, its PreviewInput, temporary-index implementation, randomUUID import, "preview" snapshot error variant, and noop entry.

Checkout history

Snapshot.checkout and Git.tree.checkout arrived in the same snapshot-system change as a legacy whole-index checkout primitive. The V2 session revert flow never called it; its only caller was the dedicated legacy checkout test. This removes the unused whole-tree chain and noop entry independently from selective restore.

Staged selective restore

Staging and clearing session reverts already build Map<RelativePath, Snapshot.ID> restore plans and call Snapshot.restore. That path remains unchanged, including per-file tree selection, deletion when a selected tree lacks a path, and containment rejection for paths escaping the project.

Persisted data

This does not change snapshot ID encoding, session/message schemas, stored revert payloads, snapshot repository layout, or captured Git trees. Existing persisted snapshot IDs remain valid inputs to files, diff, and restore; no migration or generated API update is required.

How

• Remove preview and checkout from the private Snapshot and Git tree interfaces and implementations.
• Simplify the restore planner now that it serves only restore, retaining the same containment check and Snapshot.Error mapping.
• Remove only assertions and the standalone test dedicated to the deleted APIs.

Scope

• Core is private, so no changeset is included.
• Does not modify Git.change.
• Does not modify stale move documentation.
• Does not alter session revert planning, staging, clearing, committing, projection, or persisted snapshot IDs.
• Does not change public Protocol or Server HttpApi surfaces.

Testing

• cd packages/core && bun run test test/git.test.ts test/snapshot.test.ts test/session-prompt.test.ts test/session-projector.test.ts (61 passed)
• cd packages/core && bun typecheck
• git diff --check
• Push hook: bun turbo typecheck --concurrency=3 (33 packages passed)