Add a Codex CLI provider

[Ron537]

Summary

Add a new SessionProvider for OpenAI Codex CLI so DPlex can discover and resume Codex sessions out of the box, alongside Copilot CLI and Claude Code.

Background

DPlex's provider system is documented in docs/providers.md. Each provider is one TypeScript class implementing SessionProvider (defined in src/main/services/providers/types.ts). Built-ins live in src/main/services/providers/.

Acceptance criteria

• New file src/main/services/providers/codexProvider.ts exporting a CodexProvider class that implements SessionProvider.
• Provider registers in createDefaultRegistry() in src/main/services/providers/index.ts.
• Discovers sessions from Codex's data directory (verify upstream — likely ~/.codex/sessions or similar).
• Active-session detection works (probably via lock file or pidfile, like the other providers).
• getResumeCommand(sessionId) returns the correct codex --resume=<id> shape.
• At least one unit test covering session discovery + resume command shape.

Files to touch

• src/main/services/providers/codexProvider.ts (new)
• src/main/services/providers/index.ts (register)
• tests/unit/codex-provider.test.ts (new)

How to test

npm run typecheck
npm run test:unit
npm run dev   # confirm Codex sessions appear in the Sessions tab if you have any

Reference

• See claudeCodeProvider.ts for the closest analog (also pidfile-based active detection).
• See copilotProvider.ts for the other built-in.

Effort: 1-2 days for someone unfamiliar with the codebase.