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>

mcp/llm-router/toggle.py

@@ -0,0 +1,98 @@
+#!/usr/bin/env python3
+"""
+Toggle external-only mode.
+
+Usage:
+    toggle.py on [--reason "user requested"]
+    toggle.py off
+    toggle.py status
+"""
+
+import argparse
+import json
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+STATE_FILE = Path.home() / ".claude/state/external-mode.json"
+
+
+def load_state() -> dict:
+    """Load current state."""
+    if STATE_FILE.exists():
+        with open(STATE_FILE) as f:
+            return json.load(f)
+    return {"enabled": False, "activated_at": None, "reason": None}
+
+
+def save_state(state: dict):
+    """Save state to file."""
+    with open(STATE_FILE, "w") as f:
+        json.dump(state, f, indent=2)
+
+
+def enable(reason: Optional[str] = None):
+    """Enable external-only mode."""
+    state = {
+        "enabled": True,
+        "activated_at": datetime.now().isoformat(),
+        "reason": reason or "user-requested"
+    }
+    save_state(state)
+    print("External-only mode ENABLED")
+    print(f"  Activated: {state['activated_at']}")
+    print(f"  Reason: {state['reason']}")
+    print("\nAll agent requests will now use external LLMs.")
+    print("Run 'toggle.py off' or '/pa --external off' to disable.")
+
+
+def disable():
+    """Disable external-only mode."""
+    state = {
+        "enabled": False,
+        "activated_at": None,
+        "reason": None
+    }
+    save_state(state)
+    print("External-only mode DISABLED")
+    print("\nAll agent requests will now use Claude.")
+
+
+def status():
+    """Show current mode status."""
+    state = load_state()
+    if state.get("enabled"):
+        print("External-only mode: ENABLED")
+        print(f"  Activated: {state.get('activated_at', 'unknown')}")
+        print(f"  Reason: {state.get('reason', 'unknown')}")
+    else:
+        print("External-only mode: DISABLED")
+        print("  Using Claude for all requests.")
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Toggle external-only mode")
+    subparsers = parser.add_subparsers(dest="command", required=True)
+
+    # on command
+    on_parser = subparsers.add_parser("on", help="Enable external-only mode")
+    on_parser.add_argument("--reason", help="Reason for enabling")
+
+    # off command
+    subparsers.add_parser("off", help="Disable external-only mode")
+
+    # status command
+    subparsers.add_parser("status", help="Show current mode")
+
+    args = parser.parse_args()
+
+    if args.command == "on":
+        enable(args.reason)
+    elif args.command == "off":
+        disable()
+    elif args.command == "status":
+        status()
+
+
+if __name__ == "__main__":
+    main()