fix(external-llm): remove non-existent gemini-3 models

Removed gemini/gemini-3-pro and gemini/gemini-3-flash from native
gemini CLI - these models return 404 Not Found.

Remaining gemini models (via native CLI):
- gemini/gemini-2.5-pro
- gemini/gemini-2.5-flash

Note: github-copilot/gemini-3-pro-preview (via opencode) kept for now.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

state/model-policy.json

@@ -157,18 +157,6 @@
       "use_cases": ["general"],
       "tier": "sonnet-equivalent"
     },
-    "gemini/gemini-3-pro": {
-      "cli": "gemini",
-      "cli_args": ["-m", "gemini-3-pro"],
-      "use_cases": ["long-context", "reasoning"],
-      "tier": "opus-equivalent"
-    },
-    "gemini/gemini-3-flash": {
-      "cli": "gemini",
-      "cli_args": ["-m", "gemini-3-flash"],
-      "use_cases": ["fast", "general"],
-      "tier": "sonnet-equivalent"
-    },
     "gemini/gemini-2.5-pro": {
       "cli": "gemini",
       "cli_args": ["-m", "gemini-2.5-pro"],