feat(xtask): add compare-outputs build modes

[mstykow]

Summary

• add --build-mode optimized|fast-iteration to compare-outputs, defaulting to the existing benchmark-accurate optimized build while allowing quicker ci-release reruns when only output deltas matter
• record the selected Provenant build mode/profile in compare and benchmark manifests, and update the benchmark skill plus xtask/README.md to explain when fast iteration is appropriate versus when an optimized rerun is required
• harden the xtask temp-git-repo test helper against inherited commit signing so the shared helper tests stay deterministic in local environments

Scope and exclusions

• Included: compare-outputs CLI/runtime changes, manifest metadata, xtask docs, benchmark skill guidance, and the temp-repo test helper fix uncovered during validation
• Explicit exclusions: no benchmark entry updates, no ScanCode profile changes, and no scanner-behavior changes beyond the new build-mode selection path