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/future-considerations.json

@@ -37,10 +37,11 @@
       "category": "integration",
       "title": "External LLM integration",
       "description": "Support for non-Claude models in the agent system",
-      "priority": "low",
-      "status": "deferred",
+      "priority": "medium",
+      "status": "planned",
       "created": "2024-12-28",
-      "notes": "For specialized tasks or cost optimization"
+      "plan": "gleaming-routing-mercury",
+      "notes": "Cloud APIs via subscription (Copilot, Z.AI, Gemini). Uses opencode/gemini CLIs. Session toggle for external-only mode."
     },
     {
       "id": "fc-005",