feat(checkpoint): add checkpoint list --json with --pending rewind-point view

Add machine-readable and pending-dataset modes to entire checkpoint list so the deprecated rewind --list listing can be retired (see #1767):

checkpoint list condensed checkpoints, human view (unchanged) checkpoint list --json condensed checkpoints as JSON (branchCheckpointJSON) checkpoint list --pending live shadow-branch rewind points, human list checkpoint list --pending --json rewind points as JSON — byte-for-byte the shape the removed rewind --list emitted (drop-in)

The condensed --json path reuses the existing runExplainListJSON serializer. The pending JSON shape lives in checkpoint_list.go (pendingRewindPointJSON) so it survives independently of the rewind command; the rewind-point display label is extracted into the shared rewindPointLabel used by both the interactive rewind picker and the new --pending human view. --session is rejected with --pending (the pending dataset is session-agnostic, matching rewind --list).

Removes the now-superseded rewind --list flag; the rewind action (--to/--logs-only/--reset/interactive) is unchanged.