feat: add native Gemini provider for Google AI models

Adds Google Gemini as a first-class provider using Gemini's OpenAI-compatible
endpoint, supporting gemini-2.0-flash, gemini-2.5-pro, and gemini-2.0-flash-lite
across all three model tiers (opus/sonnet/haiku).

- Add 'gemini' to APIProvider type with CLAUDE_CODE_USE_GEMINI env detection
- Map all 11 model configs to appropriate Gemini models per tier
- Route Gemini through existing OpenAI shim (generativelanguage.googleapis.com)
- Support GEMINI_API_KEY and GOOGLE_API_KEY for authentication
- Fix model display name to show actual Gemini model instead of Claude fallback
- Add Gemini support to provider-launch, provider-bootstrap, system-check scripts
- Add dev:gemini npm script for local development

Bootstrap: bun run profile:init -- --provider gemini --api-key <key>
Launch: bun run dev:gemini

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -18,6 +18,7 @@
     "dev:profile:fast": "bun run scripts/provider-launch.ts auto --fast --bare",
     "dev:codex": "bun run scripts/provider-launch.ts codex",
     "dev:openai": "bun run scripts/provider-launch.ts openai",
+    "dev:gemini": "bun run scripts/provider-launch.ts gemini",
     "dev:ollama": "bun run scripts/provider-launch.ts ollama",
     "dev:ollama:fast": "bun run scripts/provider-launch.ts ollama --fast --bare",
     "profile:init": "bun run scripts/provider-bootstrap.ts",