Add a Gemini CLI provider

[Ron537]

Summary

Add a new SessionProvider for Google Gemini CLI so DPlex can discover and resume Gemini sessions alongside Copilot CLI and Claude Code.

Acceptance criteria

• src/main/services/providers/geminiProvider.ts implementing SessionProvider.
• Registered in createDefaultRegistry() in src/main/services/providers/index.ts.
• Session discovery from Gemini's data dir (check upstream — ~/.gemini/sessions or ~/.config/google-gemini/).
• Active session detection (lock file or pidfile pattern).
• getResumeCommand(sessionId) returns the correct shape (verify with gemini --help).
• Unit test for session discovery + resume.

Files to touch

• src/main/services/providers/geminiProvider.ts (new)
• src/main/services/providers/index.ts
• tests/unit/gemini-provider.test.ts (new)

How to test

npm run typecheck && npm run test:unit && npm run dev

Reference

• docs/providers.md
• claudeCodeProvider.ts — closest pattern.

Effort: 1-2 days.