feat: gr2 exec run/build/test lane-aware execution commands

[laynepenney]

Summary

Implement the first mutating lane-aware exec commands on top of the new gr2 exec status planning surface.

Why

exec status proves scope and defaults, but the workflow is still incomplete until users and agents can actually run commands inside a selected lane without guessing repo roots, branch context, or policy defaults.

Scope

Add the first execution commands:

• gr2 exec run --lane <lane> <command>
• gr2 exec test --lane <lane>
• gr2 exec build --lane <lane>

Execution must:

• resolve repo scope from persisted lane metadata
• support repo filtering
• honor lane parallel and fail_fast defaults
• report per-repo command results and working directories
• stop short of background job orchestration or long-lived supervision

Acceptance criteria

• commands run only inside the selected lane scope
• repo filters work consistently with gr2 exec status
• result reporting makes successes/failures legible per repo
• fail-fast and parallel defaults come from lane metadata, with explicit flags able to override later

Design refs

• design: gr2 repo maintenance and multi-repo lanes #542
• feat: gr2 exec lane-aware multi-repo build/test/run surface #544
• docs: design gr2 repo maintenance and lanes #543