Add Codex CLI interpreter provider

[ch040602]

Closes #880.

Summary

• Adds Codex CLI as a local interpreter provider through browser Native Messaging.
• Adds native host setup helpers for Windows, macOS, and Linux, plus a settings-panel connection test.
• Supports existing prompt variables, including unquoted prompt syntax such as {{prompt:Return exactly OK.}}.
• Adds optional visible-tab screenshot input for Codex CLI interpreter requests.
• Falls back to an enabled interpreter model when the saved selected model is stale or empty.

Validation

• npm test -- --run src/utils/interpreter-codex.test.ts src/utils/parser.test.ts
• node native/test-codex-host.mjs
• node native/test-codex-interpreter.mjs
• npm run build:chrome

The native interpreter smoke test returned CODEX_UI_OK through the Codex CLI native host.