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/providers/opencode.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+"""OpenCode CLI wrapper for GitHub Copilot, Z.AI, and other providers."""
+
+import subprocess
+from typing import List
+
+
+def invoke(cli_args: List[str], prompt: str, timeout: int = 300) -> str:
+    """
+    Invoke opencode CLI with given args and prompt.
+
+    Args:
+        cli_args: Model args like ["-m", "github-copilot/gpt-5.2"]
+        prompt: The prompt text
+        timeout: Timeout in seconds (default 5 minutes)
+
+    Returns:
+        Model response as string
+
+    Raises:
+        RuntimeError: If opencode CLI fails
+        TimeoutError: If request exceeds timeout
+
+    Example invocation:
+        opencode -m github-copilot/gpt-5.2 -p "Hello world"
+    """
+    cmd = ["opencode"] + cli_args + ["-p", prompt]
+
+    try:
+        result = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=timeout
+        )
+    except subprocess.TimeoutExpired:
+        raise TimeoutError(f"opencode timed out after {timeout}s")
+
+    if result.returncode != 0:
+        raise RuntimeError(f"opencode failed (exit {result.returncode}): {result.stderr}")
+
+    return result.stdout.strip()
+
+
+if __name__ == "__main__":
+    # Quick test
+    import sys
+    if len(sys.argv) > 1:
+        response = invoke(["-m", "github-copilot/gpt-5.2"], sys.argv[1])
+        print(response)
+    else:
+        print("Usage: opencode.py 'prompt'")