Checkpoints v2: support attach command

[computermode]

Wires up the attach command to support checkpoints v2. Supports dual-write to v1 and v2 checkpoints when v2 is enabled.

This change is not user facing and should not introduce any changes to the checkpoints v1 attach flow.

(Used the 994182ab5aba test checkpoint in d35e512 to ensure the new v2 checkpoint flow was executed correctly via attach.)

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Note

Medium Risk
Adds conditional dual-write and backfill logic during attach, which touches git ref storage and migration paths; failures are logged but could lead to v1/v2 divergence if bugs slip through.

Overview
attach now optionally mirrors committed checkpoints into checkpoints v2 when strategy_options.checkpoints_v2 is enabled, while continuing to write to v1 as before.

When attaching to an existing v1-only checkpoint, it backfills the full checkpoint into v2 (rewriting all sessions and copying task metadata when needed) to avoid creating a partial v2 shadow; v2 write failures are treated as best-effort and only logged.

Tests add coverage for v2 dual-write output (including compact + raw transcript files in v2 refs) and for backfilling an existing v1 checkpoint after enabling v2.

^{Reviewed by Cursor Bugbot for commit 73e4598. Configure here.}

[Copilot]

Pull request overview

Wires up attach to dual-write checkpoints into the v2 ref layout when checkpoints_v2 is enabled, including a v1→v2 backfill path when attaching to an existing v1 checkpoint.

Changes:

• Extend runAttach to generate a compact transcript and best-effort dual-write to v2 when checkpoints_v2 is enabled.
• Add attach-specific v2 backfill logic to mirror existing v1-only checkpoints into v2 refs.
• Add tests covering v2 dual-write and v1-only → v2 backfill behavior for attach.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cmd/entire/cli/attach.go	Adds v2 dual-write + backfill logic for attach-created checkpoints (best-effort, non-blocking).
cmd/entire/cli/attach_test.go	Adds coverage for v2 dual-write and backfilling an existing v1-only checkpoint into v2.
test	Empty file present in the PR contents (no functional change visible from provided diff).

[pfleidi]

Thanks!