chore(cli): make campus.audit --json output shapes consistent #61

Context

The campus audit group (#55) offers --json on most commands, but the
two list-shaped commands return different shapes:

• campus audit keys list --json prints just the array of key records
  (the server's count is dropped).
• campus audit traces list --json prints the whole server response,
  including the pagination cursor.

A script that consumes both has to special-case one of them, and the
"which shape is this?" question lands on whoever writes the script.

Proposed fix

Pick one shape for the group and apply it to every --json output. The
server responses are {"api_keys": [...], "count": n} and
{"traces": [...], "cursor": {...}}; printing the server response
verbatim (the traces behaviour) keeps the count and the cursor, and stays
faithful to what the API returns.

Acceptance

• Every campus audit --json output has the same shape, decided and
  documented in the README.
• campus audit keys list --json keeps the key count.