api/checkpoint: add CompactTranscriptStart marker to session metadata

Add CompactTranscriptStart *int (compact_transcript_start) plus a GetCompactTranscriptStart accessor. transcript.jsonl now stores the full compacted session, so this marks where a checkpoint's slice begins: compactLines[CompactTranscriptStart:].

A pointer is used so "absent" (a legacy checkpoint whose transcript.jsonl holds only its delta, read from line 0) is distinguishable from 0 (a full file whose first checkpoint starts at the beginning).