feat: add exec pod grouping controls

[acmore]

Summary

• add first-class exec targeting for all pods, workers, and explicit pod groups
• support sequential or parallel group execution while preserving existing per-pod exec behavior inside each group
• write grouped log output to per-group subdirectories and document the new exec orchestration flags

Verification

• go test ./internal/cli
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt