explain: enrich JSON summary and surface list truncation
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Commit

- --json now exports the full persisted session summary (friction, open_items, and categorized learnings), not just intent/outcome, so scripts and dashboards see what the prose view already renders
- the prose branch list notes on stderr when checkpoints are hidden beyond the 100 cap, pointing at --json --limit; wording stays honest ("more exist", never an exact remaining count)
- --generate on imported checkpoints now fails fast on metadata alone, before the transcript content load
- clearer no-Entire-trailer message distinguishing a found-but-untracked commit from a missing one
Entire-Checkpoint: 7460467d8fa2
Checkpoints
Checkpoint 1