Checkpoints V2: Support checkpoint_transcript_start for compact transcript.jsonl files

[computermode]

Ensures we have checkpoint_transcript_start in the metadata.json for the compact transcript.jsonl files and that we append to the transcript.jsonl for the corresponding session with each subsequent checkpoint (without deleting/cutting information from the file).

Also updates the migrate command to respect the start lines (previously, if I migrated, then added v1 checkpoints and migrated again, it wouldn't calculate checkpoint_transcript_start).

I tested this by creating multiple commits and checkpoints with a Claude session and inspecting the v2 refs to ensure that the checkpoint_transcript_start lines were added to the metadata.json file as expected. I also wired up the UI to read from v2 transcript.jsonl and ensured that only the new relevant logs to each checkpoint were displayed in the UI.

Testing with Codex next...

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Note

Medium Risk
Changes how checkpoint_transcript_start is computed/persisted for v2 /main metadata and updates session state/migration logic to maintain a separate compact-transcript offset; mistakes could break transcript slicing or backfills across checkpoints.

Overview
V2 checkpoint metadata now records checkpoint_transcript_start in the compact transcript domain (transcript.jsonl) instead of the full transcript (full.jsonl). This adds CompactTranscriptStart to WriteCommittedOptions and updates V2GitStore to write that value into CommittedMetadata.CheckpointTranscriptStart on /main.

Manual-commit condensation now generates cumulative transcript.jsonl for v2 and tracks compact offsets across condensations. Session state gains compact_transcript_start, condensation computes/falls back to a compact start offset when missing, and state updates accumulate compact line counts so subsequent checkpoints get correct offsets.

migrate --checkpoints v2 now computes and writes the compact start offset during migration/backfill. It derives the offset by diffing full vs scoped compactions and includes new tests covering v2 write behavior, condensation behavior, and migration correctness.

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[Copilot]

Pull request overview

Adds support for correctly tracking checkpoint_transcript_start for v2 compact transcript.jsonl artifacts by introducing a compact-transcript-specific offset and propagating it through condensation and migration.

Changes:

• Introduces CompactTranscriptStart/CompactTranscriptLines plumbing to track compact transcript offsets separately from full.jsonl offsets.
• Updates v2 /main committed metadata writing to use the compact transcript start offset for checkpoint_transcript_start.
• Enhances migrate to compute and persist compact transcript offsets when generating transcript.jsonl.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cmd/entire/cli/strategy/manual_commit_types.go	Extends condensation result to report compact transcript line counts.
cmd/entire/cli/strategy/manual_commit_test.go	Adds coverage ensuring v2 /main writes checkpoint_transcript_start correctly for compact transcripts.
cmd/entire/cli/strategy/manual_commit_hooks.go	Updates session state to advance/reset compact transcript offsets across condensations/carry-forward.
cmd/entire/cli/strategy/manual_commit_condensation.go	Plumbs compact transcript start into write options and computes compact transcript line deltas.
cmd/entire/cli/session/state.go	Adds persisted compact_transcript_start to session state with legacy backfill behavior.
cmd/entire/cli/session/state_test.go	Adds tests for CompactTranscriptStart normalization/backfill and JSON round-trip.
cmd/entire/cli/migrate.go	Computes compact transcript offsets during migration and stores them in v2 write options.
cmd/entire/cli/checkpoint/v2_store_test.go	Tests that v2 /main metadata uses CompactTranscriptStart for checkpoint_transcript_start.
cmd/entire/cli/checkpoint/v2_committed.go	Switches v2 /main metadata field to use compact transcript start offset.
cmd/entire/cli/checkpoint/checkpoint.go	Adds CompactTranscriptStart to WriteCommittedOptions for v2 metadata writing.

[computermode]

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^{Reviewed by Cursor Bugbot for commit 6607a7e. Configure here.}