Complete migration from old-style user tools (tools/*.py) to MCP servers.

Profile changes:
- Replace write_tool, delete_tool, test_tool with create_mcp_server,
  test_mcp_tool, mcp_status, reload_tools
- Update system_prompt.txt with 10-step MCP workflow
- Rewrite subagent_system_prompt.txt with canonical server format,
  absolute rules, smoke-test procedures
- Remove mcp_servers mapping from config (no longer bound to
  external servers)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>