Add hidden `entire checkpoint create` for snapshot checkpoints

Creates a checkpoint from a session's current transcript and prints its ID. With no argument it checkpoints the calling agent's session, and only when the caller was identified (caller-env or ancestry); most-recent fallbacks are refused so a guess cannot checkpoint someone else's work.

The write goes through CondenseSession, so extraction, redaction, and the store write (push-queue enqueue / unpushed v1 commit) are unchanged. It is a snapshot: state is loaded, condensed in memory, and never saved back, so a mid-turn call cannot race the session's hooks and the next commit still condenses the same range. Skill telemetry is not emitted, since the next commit would count those events again.

Commit attribution is omitted (new condenseOpts.noCommitAttribution): it diffs the shadow tree against HEAD assuming HEAD holds the committed work, which without a commit counts uncommitted agent edits as human edits.