fix: respect WORKSPACE_MCP_CREDENTIALS_DIR for multi-account support

- Add WORKSPACE_MCP_CREDENTIALS_DIR as primary env var (preferred)
- Keep GOOGLE_MCP_CREDENTIALS_DIR for backward compatibility
- Add os.path.expanduser() to handle ~ in paths
- Add logging to show which credentials directory is being used
- Display credentials directory in startup configuration

This enables running multiple MCP instances with different Google
accounts by configuring separate credential directories.

Fixes #373

main.py

@@ -153,10 +153,26 @@ def main():
         else "Invalid or too short"
     )
 
+    # Determine credentials directory (same logic as credential_store.py)
+    workspace_creds_dir = os.getenv("WORKSPACE_MCP_CREDENTIALS_DIR")
+    google_creds_dir = os.getenv("GOOGLE_MCP_CREDENTIALS_DIR")
+    if workspace_creds_dir:
+        creds_dir_display = os.path.expanduser(workspace_creds_dir)
+        creds_dir_source = "WORKSPACE_MCP_CREDENTIALS_DIR"
+    elif google_creds_dir:
+        creds_dir_display = os.path.expanduser(google_creds_dir)
+        creds_dir_source = "GOOGLE_MCP_CREDENTIALS_DIR"
+    else:
+        creds_dir_display = os.path.join(
+            os.path.expanduser("~"), ".google_workspace_mcp", "credentials"
+        )
+        creds_dir_source = "default"
+
     config_vars = {
         "GOOGLE_OAUTH_CLIENT_ID": os.getenv("GOOGLE_OAUTH_CLIENT_ID", "Not Set"),
         "GOOGLE_OAUTH_CLIENT_SECRET": redacted_secret,
         "USER_GOOGLE_EMAIL": os.getenv("USER_GOOGLE_EMAIL", "Not Set"),
+        "CREDENTIALS_DIR": f"{creds_dir_display} ({creds_dir_source})",
         "MCP_SINGLE_USER_MODE": os.getenv("MCP_SINGLE_USER_MODE", "false"),
         "MCP_ENABLE_OAUTH21": os.getenv("MCP_ENABLE_OAUTH21", "false"),
         "WORKSPACE_MCP_STATELESS_MODE": os.getenv(