feat: gr2 lane checkout-pr for isolated multi-repo review lanes

[laynepenney]

Summary

Implement gr2 lane checkout-pr so users can review or patch a PR in a disposable review lane without disturbing active feature work.

Why

Review isolation is a core requirement for both human and agent workflows. A PR checkout must not mutate the current feature lane.

Scope

• create or update a review lane from a PR
• record PR association in lane metadata
• allow lane-local build/test/run after checkout
• keep review lanes disposable and clearly typed

Acceptance criteria

• checking out a PR does not disturb the current feature lane
• review lanes can coexist with feature and scratch lanes
• PR association is durable in metadata
• behavior works for cross-repo and single-repo review flows

Design refs

• design: gr2 repo maintenance and multi-repo lanes #542
• docs: design gr2 repo maintenance and lanes #543