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.

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.

Testing with Codex next...

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Note

Medium Risk
Adjusts how checkpoint_transcript_start is computed and stored for v2 /main and during migration, which can affect transcript scoping and downstream consumers if offsets are wrong. Changes are localized and covered by new tests, but touch persistence/state and migration behavior.

Overview
Ensures v2 /main metadata writes checkpoint_transcript_start in the compact transcript.jsonl line domain by adding CompactTranscriptStart to WriteCommittedOptions and using it in v2_committed.go.

Updates manual-commit condensation and session state to track compact_transcript_start, compute/fallback the compact offset when missing, and advance it after each condensation so subsequent checkpoints are correctly scoped.

Improves migrate --checkpoints v2 to compute and persist compact transcript start offsets when generating transcript.jsonl, with new unit tests validating v2 metadata uses the compact offset (not full.jsonl offsets) across write, condense, and migration paths.

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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]

bugbot review

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