feat(external-llm): add external LLM integration (fc-004)

Implements external LLM routing via opencode CLI for:
- GitHub Copilot (gpt-5.2, claude-sonnet-4.5, claude-haiku-4.5, o3, gemini-3-pro)
- Z.AI (glm-4.7 for code generation)
- OpenCode native (big-pickle)

Components:
- mcp/llm-router/invoke.py: Main router with task-based model selection
- mcp/llm-router/delegate.py: Agent delegation helper (respects external mode)
- mcp/llm-router/toggle.py: Enable/disable external-only mode
- mcp/llm-router/providers/: CLI wrappers for opencode and gemini

Features:
- Persistent toggle via state/external-mode.json
- Task routing: reasoning -> gpt-5.2, code-gen -> glm-4.7, long-context -> gemini
- Claude tier mapping: opus -> gpt-5.2, sonnet -> claude-sonnet-4.5, haiku -> claude-haiku-4.5
- Session-start hook announces when external mode is active
- Natural language toggle support via component registry

Plan: gleaming-routing-mercury

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

state/model-policy.json

@@ -113,5 +113,60 @@
       "default": "haiku",
       "escalate_on": ["insufficient_context", "reasoning_required", "user_dissatisfied"]
     }
+  },
+  "external_models": {
+    "github-copilot/gpt-5.2": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/gpt-5.2"],
+      "use_cases": ["reasoning", "fallback"],
+      "tier": "opus-equivalent"
+    },
+    "github-copilot/claude-sonnet-4.5": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/claude-sonnet-4.5"],
+      "use_cases": ["general", "fallback"],
+      "tier": "sonnet-equivalent"
+    },
+    "github-copilot/claude-haiku-4.5": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/claude-haiku-4.5"],
+      "use_cases": ["simple"],
+      "tier": "haiku-equivalent"
+    },
+    "zai-coding-plan/glm-4.7": {
+      "cli": "opencode",
+      "cli_args": ["-m", "zai-coding-plan/glm-4.7"],
+      "use_cases": ["code-generation"],
+      "tier": "sonnet-equivalent"
+    },
+    "github-copilot/gemini-3-pro-preview": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/gemini-3-pro-preview"],
+      "use_cases": ["long-context"],
+      "tier": "opus-equivalent"
+    },
+    "github-copilot/o3": {
+      "cli": "opencode",
+      "cli_args": ["-m", "github-copilot/o3"],
+      "use_cases": ["complex-reasoning"],
+      "tier": "opus-equivalent"
+    },
+    "opencode/big-pickle": {
+      "cli": "opencode",
+      "cli_args": ["-m", "opencode/big-pickle"],
+      "use_cases": ["general"],
+      "tier": "sonnet-equivalent"
+    }
+  },
+  "claude_to_external_map": {
+    "opus": "github-copilot/gpt-5.2",
+    "sonnet": "github-copilot/claude-sonnet-4.5",
+    "haiku": "github-copilot/claude-haiku-4.5"
+  },
+  "task_routing": {
+    "reasoning": "github-copilot/gpt-5.2",
+    "code-generation": "zai-coding-plan/glm-4.7",
+    "long-context": "github-copilot/gemini-3-pro-preview",
+    "default": "github-copilot/claude-sonnet-4.5"
   }
 }