feat(external-llm): add gemini-3-flash-preview via OpenCode

Gemini 3 models via github-copilot provider (OpenCode):
- github-copilot/gemini-3-pro-preview (opus-tier)
- github-copilot/gemini-3-flash-preview (sonnet-tier)

Native Gemini CLI models unchanged:
- gemini/gemini-2.5-pro
- gemini/gemini-2.5-flash

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

state/model-policy.json

@@ -142,9 +142,15 @@
     "github-copilot/gemini-3-pro-preview": {
       "cli": "opencode",
       "cli_args": ["-m", "github-copilot/gemini-3-pro-preview"],
-      "use_cases": ["long-context"],
+      "use_cases": ["long-context", "reasoning"],
       "tier": "opus-equivalent"
     },
+    "github-copilot/gemini-3-flash-preview": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/gemini-3-flash-preview"],
+      "use_cases": ["fast", "general"],
+      "tier": "sonnet-equivalent"
+    },
     "github-copilot/o3": {
       "cli": "opencode",
       "cli_args": ["-m", "github-copilot/o3"],