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/component-registry.json

@@ -143,6 +143,23 @@
         "block dangerous",
         "protect"
       ]
+    },
+    "external-mode": {
+      "description": "Toggle between Claude and external LLMs (Copilot, Z.AI, Gemini)",
+      "script": "~/.claude/mcp/llm-router/toggle.py",
+      "triggers": [
+        "external",
+        "use external",
+        "switch to external",
+        "external models",
+        "external only",
+        "use copilot",
+        "use opencode",
+        "back to claude",
+        "use claude again",
+        "disable external",
+        "external mode"
+      ]
     }
   },
   "commands": {