Set up and fix oahelper-mcp

Part of oahelper-mcp by Sirjan Singh.

The session cookie is a password. Never ask the user to paste it into chat, never print .env, and never echo a header value back. Check whether a key is set, not what it is.

1. Find the install

Work out which way it was installed:

• Claude Code plugin (oahelper@oahelper-mcp in claude plugin list): the cookie is set with /plugin configure oahelper@oahelper-mcp.
• uvx (uvx ... oahelper-mcp in the client config): data folder ~/.oahelper unless OAHELPER_HOME is set.
• Clone (python .../server.py): data folder is the clone itself.

oahelper_status() prints the data folder when the server is reachable.

2. Diagnose in this order, stop at the first failure