Description:
The rpg-mcp-server currently receives its project root as a CLI argument. When MCP clients (like Antigravity or opencode) pass ${workspaceFolder} without expanding it, the server receives the literal string and fails to find any source files.

This PR adds a resolve_project_root function that:

• Detects ${workspaceFolder} in the CLI argument
• Resolves it to std::env::current_dir() (the MCP client launches the server in the workspace directory)
• Handles partial patterns like ${workspaceFolder}/subdir
• Logs the resolution to stderr so users know what happened

This allows the rpg-mcp-server to be configured in global MCP configs rather than requiring per-project local configs with hardcoded paths.