Add a configuration key-path parity gate for Base Registry Engine

Found in the 0.28.0 pre-release review (2026-09-08). Line references are as of main after #926.

Evidence has `products/evidence/scripts/check-config-key-paths.sh`, which keeps every configuration reference in exact parity with the schema it explains. BReg has no analogue: no `products/breg/contracts/*.yaml` enumerates a configuration key, and since #927 stopped committing the generated `breg-configuration.json`, nothing outside the generator detects a silently dropped key. Add the same gate for the registry-project, module and runtime schemas, and wire it into CI next to the Evidence one.

Start by comparing products/evidence/scripts/check-config-key-paths.sh with the Base Registry Engine configuration schemas under products/breg/contracts/*.yaml, then locate the generator that previously produced breg-configuration.json. Extend the parity coverage to the registry-project, module, and runtime schemas, and wire the check into CI next to the Evidence gate. Done means CI detects any configuration key missing from its schema.

Written by the indexing model from the issue text.