fix(external-llm): correct gemini CLI model tiers

- gemini/gemini-2.5-pro: opus -> sonnet-equivalent
- gemini/gemini-2.5-flash: sonnet -> haiku-equivalent

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

state/model-policy.json

@@ -167,13 +167,13 @@
       "cli": "gemini",
       "cli_args": ["-m", "gemini-2.5-pro"],
       "use_cases": ["long-context", "reasoning"],
-      "tier": "opus-equivalent"
+      "tier": "sonnet-equivalent"
     },
     "gemini/gemini-2.5-flash": {
       "cli": "gemini",
       "cli_args": ["-m", "gemini-2.5-flash"],
       "use_cases": ["fast", "general"],
-      "tier": "sonnet-equivalent"
+      "tier": "haiku-equivalent"
     }
   },
   "claude_to_external_map": {