feat: generation rotation for /full/current ref

[pfleidi]

Summary

Implements generational rotation for the v2 /full/current ref (task A6 from the checkpoints v2 design spec). When /full/current accumulates >= 100 checkpoints, it is archived as a numbered ref and a fresh orphan is created.

• generation.json at tree root tracks checkpoint IDs and timestamps, updated on every WriteCommitted (skipped for UpdateCommitted since that's stop-time finalization, not a new checkpoint)
• 2-phase rotation: archive current ref as refs/entire/checkpoints/v2/full/0000000000001, create fresh orphan /full/current with seed generation.json
• Non-fatal rotation — if rotation fails, the checkpoint write already succeeded; next writer retries
• Concurrent rotation guard — re-reads /full/current before archiving to skip if another instance already rotated
• Dropped deprecated TranscriptLinesAtStart from v2 metadata writes (v2 is a clean break)

New files

• checkpoint/v2_generation.go — GenerationMetadata type, read/write helpers, rotation, archived generation listing
• checkpoint/v2_generation_test.go — 17 unit tests

Modified files

• paths/paths.go — V2FullRefPrefix, GenerationFileName constants
• checkpoint/v2_store.go — MaxCheckpointsPerGeneration field for testability
• checkpoint/v2_committed.go — generation tracking + rotation check wired into write path
• checkpoint/v2_store_test.go — 2 integration tests for rotation triggering

Test plan

• Unit tests cover: read/write round-trips, accumulation, dedup, tree preservation, archive listing, sequential numbering, rotation mechanics, concurrent rotation guard, threshold triggering, below-threshold no-op
• mise run test:ci passes (unit + integration + E2E canary)
• mise run lint clean

🤖 Generated with Claude Code

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Note

Medium Risk
Adds automatic archival/rotation of the v2 refs/entire/checkpoints/v2/full/current ref based on checkpoint count, which changes git ref/tree update behavior and could impact transcript retention if bugs exist. Risk is mitigated by extensive new unit/integration tests and making rotation non-fatal to the write path.

Overview
Adds generation tracking + rotation for v2 raw transcript storage under /full/current: each WriteCommitted now updates a root generation.json (checkpoint IDs + oldest/newest timestamps) and, once the checkpoint count hits a threshold (default 100), archives the current ref as a numbered refs/entire/checkpoints/v2/full/<NNN...> and resets /full/current to a fresh orphan commit.

Introduces new generation/rotation helpers on V2GitStore (including archived generation discovery and sequential numbering), a test-only MaxCheckpointsPerGeneration override, and new path constants (V2FullRefPrefix, GenerationFileName). Also drops writing the deprecated TranscriptLinesAtStart field in v2 committed metadata and adds comprehensive tests covering metadata updates, deduping, rotation mechanics, and threshold behavior.

^{Written by Cursor Bugbot for commit 3dd257d. Configure here.}

[Copilot]

Pull request overview

Adds generational rotation to the v2 /full/current checkpoint ref so large transcript histories are periodically archived into numbered refs, keeping /full/current bounded while preserving access to older generations.

Changes:

• Introduces generation.json metadata at the root of each /full/* generation and updates it on each WriteCommitted.
• Implements rotation: archive /full/current to a numbered ref and reset /full/current to a fresh orphan commit when the checkpoint threshold is reached.
• Adds unit/integration tests validating generation metadata behavior, archived generation discovery, numbering, and rotation triggering.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
cmd/entire/cli/paths/paths.go	Adds constants for /full/* ref prefix and generation.json filename.
cmd/entire/cli/checkpoint/v2_store.go	Adds a test override for the max checkpoints-per-generation threshold.
cmd/entire/cli/checkpoint/v2_committed.go	Wires generation metadata updates and post-write rotation checks into /full/current writes.
cmd/entire/cli/checkpoint/v2_generation.go	Implements generation metadata read/write, archived generation listing, numbering, and rotation logic.
cmd/entire/cli/checkpoint/v2_generation_test.go	Adds unit tests covering generation read/write, accumulation/dedup, listing, and rotation behavior.
cmd/entire/cli/checkpoint/v2_store_test.go	Adds tests asserting rotation triggers at threshold and not below it.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.