tests: add CLI smoke tests for auth/config commands

[shrey16]

Goal

Add CLI smoke tests for the auth/config commands that are easiest to regress.

Suggested coverage

• bharatcode --help includes BharatCode-first copy.
• bharatcode doctor reports missing auth without crashing.
• config command preserves existing plugin entries and appends bharatcode.
• launch path clears explicit API-key env vars before spawning the engine.

Acceptance

• Tests run with npm test.
• Tests do not open a real browser or require real OAuth credentials.
• No secrets or private paths are committed.