Fix Codex ceremony budget prompt parity

[tmusser]

Summary

• pass the catalogued ceremony-budget protocol into Codex prompt rendering;
• preserve the distinction between an unset budget and an explicitly empty override;
• align Claude and Codex ceremony-budget prompt behavior;
• add regression tests for budget-on, budget-off, prompt isolation, and unrelated tasks.

Validation

• benchmark_harness/tests/test_ceremony_budget.py: 8 passed
• full harness suite: 258 passed
• shell syntax checks: passed
• git diff --check: passed

Scope

This PR contains only the runner parity repair and its tests. Benchmark bundles and the single-pair experimental report remain local and are not included.