[codex] Add inference policy trace metadata

[realadeel]

Summary

Makes inference routing policy-mediated and observable.

The router now checks policy-engine for inference.local or inference.cloud before executing a selected backend, emits policy decision audit events, and attaches a structured trace with route reasons, usage, latency, and configurable cost estimates.

Impact

• Adds policy checks for selected inference backends.
• Falls back from cloud to local when cloud policy denies and a local model is available.
• Blocks local-to-cloud fallback when cloud policy denies.
• Adds request ids, policy decisions, route decisions, token usage, latency, and cost estimate fields to inference results.
• Extends inference dashboard stats with policy denials and cloud cost counters.

Validation

• python -m pytest tests/ -q -> 117 passed