fix(external-llm): correct opencode CLI syntax and gemini routing

- OpenCode: use `opencode run -m MODEL "prompt"` syntax
- OpenCode: set correct binary path (/home/linuxbrew/.linuxbrew/bin/opencode)
- Gemini: route long-context to gemini-2.5-pro (gemini-3 not available yet)

Tested working:
- opencode/big-pickle
- github-copilot/claude-sonnet-4.5
- zai-coding-plan/glm-4.7
- gemini/gemini-2.5-pro
- gemini/gemini-2.5-flash

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

state/model-policy.json

@@ -190,7 +190,7 @@
   "task_routing": {
     "reasoning": "github-copilot/gpt-5.2",
     "code-generation": "zai-coding-plan/glm-4.7",
-    "long-context": "gemini/gemini-3-pro",
+    "long-context": "gemini/gemini-2.5-pro",
     "default": "github-copilot/claude-sonnet-4.5"
   }
 }