Commands can now delegate to a running Arc API server instead of
executing in-process. Adds ExecutionMode, ServerDefaults, and
ClientTlsConfig to cli.toml parsing with CLI flag > config > default
precedence. The models list command fetches from GET /models when in
server mode, with mTLS client certificate auth when [server.tls] is
configured.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>