chore(cli): auth logout should mention the stored campus.audit key #58

Context

The campus.audit API key (#54, #55) rides the same credential store as
the OAuth tokens, but under its own name — it is a separate credential with
a separate lifetime (no refresh flow; it is cleared by campus audit logout or replaced by a new campus audit login).

campus auth logout clears the OAuth tokens and reports "Logged out
successfully." It says nothing about the audit key, which is still sitting
in the store. A user who signs out expecting every stored credential to be
gone is left with a live API key they did not know about.

Proposed fix

When a campus.audit key is still stored, campus auth logout prints a
one-line note naming it and the command that clears it:

Note: the stored campus.audit API key is unchanged; run 'campus audit
logout' to clear it.

No note when no audit key is stored, and an unreadable credential store
must not break the logout itself.

Acceptance

• campus auth logout with a stored audit key prints the note naming
  campus audit logout.
• campus auth logout without a stored audit key prints no note.
• A credential store that raises on read does not fail the logout.