feat(providers): add Codex CLI support

[Orinks]

Summary

• add Codex CLI as a built-in local tool-use provider alongside Claude CLI
• add Codex CLI detection, settings UI status, provider metadata, and model routing
• implement a Codex JSONL runner that maps codex exec events into Novel Engine stream events and tracks changed files
• update README, user guide, changelog, and architecture docs

Verification

• npm install --ignore-scripts
• npm run lint

Notes

• Full npm install failed on this Windows machine because better-sqlite3 had no Node 24 prebuilt binary and the Visual Studio ClangCL toolset is not installed. Typecheck was verified after installing dependencies with lifecycle scripts disabled.