[Topherhindman]

Fixes DEVX-902

#1084 made --dev with an API key or secret an error, but the check is in project resolution. lk agent dev and lk agent start skip project resolution when the URL, API key and secret are all given, from flags or from LIVEKIT_URL, LIVEKIT_API_KEY and LIVEKIT_API_SECRET. So in a shell with all three set, --dev was ignored and the agent got the env's server and key:

$ LIVEKIT_URL=wss://env.livekit.cloud LIVEKIT_API_KEY=APIenv LIVEKIT_API_SECRET=secretenv lk agent start --dev
Detected Node.js agent (src/main.js in ./my-agent)
agent got LIVEKIT_URL=wss://env.livekit.cloud LIVEKIT_API_KEY=APIenv

With only some of them set, the agent commands already failed with #1084's error. Now they fail with a full set too:

$ LIVEKIT_URL=wss://env.livekit.cloud LIVEKIT_API_KEY=APIenv LIVEKIT_API_SECRET=secretenv lk agent start --dev
Detected Node.js agent (src/main.js in ./my-agent)
--dev can't be combined with API key credentials; unset LIVEKIT_API_KEY and LIVEKIT_API_SECRET or drop --dev

It says "remove --api-key and --api-secret" when the values come from the flags. The debugger's --audio run uses the same code, so it gets the fix too. --dev alone, or with --url, is unchanged, and so is a full set of credentials without --dev.

Tests:

• TestResolveCredentials_DevWithCredentialsIsAnError: a full set from env, a full set from flags, and an env key alone all fail with the message. The first two fail on main.
• TestResolveCredentials_DevAlone: --dev and --dev --url still give the agent devkey and secret.
• A script test (agent-dev-with-env-credentials.txtar): lk agent dev --dev and lk agent start --dev with all three env vars, and lk agent start --dev with all three flags, fail with the message and send no request. It fails on main.
• go build ./..., go vet ./... and go test ./... pass.