Add Codex CLI provider

[SIGMAREAL]

Summary

• add a local Codex CLI provider preset that uses the existing codex login instead of an API key
• add a Tauri subprocess transport for codex exec --json with detect/spawn/kill commands
• wire the frontend streaming path, settings status pill, provider typing, and no-key provider checks

Testing

• npm run typecheck
• npm run test:mocks -- --run src/lib/codex-cli-transport.test.ts src/lib/has-usable-llm.test.ts src/lib/tests/llm-providers.test.ts
• npm run build

Rust build was not run locally because cargo is not installed in this environment.