fix: resolve read-only file system issues for Nix packaging

- Add config_paths.py for XDG-compliant path resolution
- Update all servers to use user-writable locations (logs, tokens, .env)
- Make .env file optional - server works with environment variables only
- Default paths: ~/.local/share for data, ~/.local/state for logs
- Update Nix wrapper with XDG defaults and helpful postInstall message
- Update README.md with environment variable configuration documentation

Fixes OSError when running in Nix store (read-only filesystem)

token_storage.py

@@ -12,12 +12,12 @@ import threading
 from datetime import datetime, timedelta
 import logging
 
-# Determine the directory where this script (token_storage.py) is located
+from config_paths import get_token_file_path
+
 SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
-# Allow token file location to be configured via environment variable
-# Falls back to oauth_tokens.json in the script directory if not set
 TOKEN_FILE = os.environ.get(
-    "BASECAMP_TOKEN_FILE", os.path.join(SCRIPT_DIR, "oauth_tokens.json")
+    "BASECAMP_TOKEN_FILE",
+    str(get_token_file_path()),
 )
 
 # Lock for thread-safe operations