Add runtime environment variable overrides for provider configuration

[daniel-salib]

Allows overriding provider base_url and wire_api via environment variables:

• CODEX_PROVIDER_<PROVIDER_ID>_BASE_URL
• CODEX_PROVIDER_<PROVIDER_ID>_WIRE_API

Where <PROVIDER_ID> is the provider ID from config.toml, uppercased with hyphens replaced by underscores (e.g., 'vllm-local' -> 'VLLM_LOCAL').

This enables testing against different backend servers without modifying config files, useful for:

• Testing different ports (http://localhost:8000 vs 8001)
• Switching between environments (dev/staging/prod)
• CI/CD pipelines with dynamic configuration

Example usage:
CODEX_PROVIDER_VLLM_LOCAL_BASE_URL="http://localhost:8001/v1" \ CODEX_PROVIDER_VLLM_LOCAL_WIRE_API="responses" \ codex exec "what is 2+2"

Implementation details in RUNTIME_PROVIDER_OVERRIDES.md

[github-actions]

Closing this pull request because it has had no updates for more than 14 days. If you plan to continue working on it, feel free to reopen or open a new PR.