Add fast path checks for checkpoints migrations

[computermode]

https://entire.io/gh/entireio/cli/trails/289

Speed up entire migrate --checkpoints v2 reruns by short-circuiting checkpoints that are already fully migrated and avoiding redundant v1 reads for permanently unmigratable compact transcripts.

• Add a fast-path guard in migrateOneCheckpoint that returns errAlreadyMigrated when the v2 checkpoint already has a compact transcript and /full/<n> artifacts for every session
• In backfillCompactTranscripts, read the v2 session metadata first and skip the v1 transcript decode when Agent == "" (compact is permanently unrecoverable for those sessions)

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Note

Medium Risk
Changes checkpoint migration control-flow to short-circuit work on reruns, which could affect whether repairs/backfills run for partially migrated data. Covered by new tests, but it still touches a data-migration path.

Overview
Speeds up entire migrate --checkpoints v2 reruns by adding a fast-path in migrateOneCheckpoint that immediately skips checkpoints already fully migrated (all sessions have compact transcripts and archived /full/<n> artifacts).

Optimizes compact transcript backfill by first checking v2 session metadata and skipping v1 reads for sessions with no recorded agent type (permanently non-generatable compact transcripts). Adds focused tests for the new predicate, fast-path behavior, and the no-agent skip case.

^{Reviewed by Cursor Bugbot for commit 7d09b51. Configure here.}

[Copilot]

Pull request overview

Speeds up entire migrate --checkpoints v2 reruns by short-circuiting checkpoints that already look fully migrated and by avoiding unnecessary v1 reads during compact-transcript backfill. This fits into the migration path for the CLI’s v1→v2 checkpoint storage.

Changes:

• Added a fast-path in migrateOneCheckpoint to skip already-complete v2 checkpoints.
• Added v2CheckpointFullyMigrated to detect when compact transcripts and /full/* artifacts already exist.
• Optimized compact-transcript backfill to inspect v2 metadata first and skip v1 transcript reads for sessions with no recorded agent.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cmd/entire/cli/migrate.go	Adds the new rerun fast-path and the metadata-first backfill shortcut.
cmd/entire/cli/migrate_test.go	Adds tests for the fast-path helper and the no-agent backfill shortcut.