Allow API-key auth for remote exec-server registration

[sdcoffey]

Overview

Allow remote codex exec-server registration to use existing API-key auth for remotes that accept API keys.

• Accept CodexAuth::ApiKey for the normal --remote registration path while retaining --use-agent-identity-auth as the explicit Agent Identity path.
• Document remote registration using CODEX_API_KEY.

Big picture

Callers can now provide an API key directly to exec-server registration without first establishing ChatGPT login state:

CODEX_API_KEY="$OPENAI_API_KEY" \
codex exec-server \
  --remote https://api.example.com/api \
  --environment-id "$ENVIRONMENT_ID"

Validation

• cargo fmt --all -- --check (just fmt is not installed on this host)
• cargo test -p codex-cli -p codex-exec-server (run before the documentation and test-only cleanup; the behavioral code is unchanged)