fix: add --quiet to create-operator-key for scripting

[aleksUIX]

Summary

Fixes #79. ad-seller create-operator-key had no machine-readable mode, so bootstrap scripts had to scrape the minted key out of the rich panel. ad-buyer create-operator-key already has --quiet / -q.

This adds the same flag: print only response.api_key and return before the panel. Stdout is flushed so the value survives command substitution.

KEY=$(ad-seller create-operator-key --label ci --quiet)

Default interactive output is unchanged.

Test plan

• uv run --locked pytest tests/unit/test_operator_auth.py::TestCliBootstrap -q (7 passed)
• uv run --locked ruff check / ruff format on the touched files

[Sirajmx]

Verified: unit + integration passed (0 failed), merged onto current main. Small,
exactly matches the buyer-agent's existing --quiet pattern, default interactive output
unchanged. No conflicts merging onto main.

[atc964]

Approved. Thanks for turning this around so quickly.

Verified the implementation matches the buyer agent's --quiet behaviour: bare key
to stdout, flush=True so it survives command substitution, early return before the
panel, and the default interactive output untouched. The test asserting stdout is
exactly the key and stderr is empty is the right assertion for this.

Two non-blocking nits, neither worth holding the merge for:

1. if quiet is True: can just be if quiet:.
2. In docs/api/authentication.md the two commands now sit in one block and read as a
   sequence to run in order, which would mint two separate operator keys. Splitting
   them, or showing the --quiet form as an alternative rather than a follow-on, would
   read more clearly.

Happy for this to merge as-is.