Harden enterprise runtime controls

[AdamDiStefanoAI]

Summary

• implement the modify decision end to end across policy evaluation, gateway responses, telemetry, and persistence
• enforce authoritative spend controls for both direct execution and approval-based execution, with agent boundary support for default and per-tool costs
• add signed audit/evidence integrity, tighten production secret requirements, and stop persisting operator secrets in browser local storage
• expand and stabilize the test suite so the non-integration gate passes again

Verification

• PYTHONPATH='src:.' JWT_SECRET_KEY='ci_test_secret_not_for_production_at_all' KILLSWITCH_SECRET='ci_killswitch_secret_not_for_production' ACR_ENV='test' pytest tests/ -v --ignore=tests/integration --cov=src/acr --cov-report=term-missing --cov-fail-under=80
• result: 200 passed, coverage 80.19%

Notes

• adds Alembic migration 0013_policy_decision_modify
• unrelated local changes under implementations/acr-control-plane/var/policy_bundles/... were intentionally left out of this branch