agents: state the codex bridge's contract in its description

The description a selector reads said only "Bridge one structured
code-verifier task to Codex" — nothing about what goes in, what comes
back, or why to pick it over code-verifier, which is the one distinction
that matters between the two. Say the contract instead, avoiding ": " so
strict YAML parsers accept the frontmatter.

The usbmon description loses its trailing list of example phrasings;
its first sentence already names those intents, and descriptions are
paid for on every request.