Send claude-cli User-Agent for anthropic mutate to bypass proxy cloaking

The CLIProxyAPI proxy cloaks requests from non-Claude-Code clients by
replacing the system prompt with Claude Code's prompts and shoving the
user's sys_msg into the first user message. Sending a User-Agent that
starts with 'claude-cli' makes ShouldCloak return false, so user
sys_msg passes through untouched.

Only applied to the anthropic override; openai/gemini are unaffected.

Author: Sixzero

src/OpenRouterCLIProxyAPI.jl

@@ -184,6 +184,11 @@ end
 Override providers to route through cli_proxy_api.
 Set `gemini=true` to also override google-ai-studio.
 """
+# User-Agent prefix that CLIProxyAPI recognizes as real Claude Code
+# (see helps.ShouldCloak: `strings.HasPrefix(userAgent, "claude-cli")`).
+# Sending this bypasses the proxy's system-prompt cloaking so user sys_msg passes through.
+const CLAUDE_CLI_USER_AGENT = "claude-cli/2.1.63 (external, cli)"
+
 function override_providers!(base_url::String, api_key_env_var::String;
                              gemini::Bool=false, verbose::Bool=false)
     anthropic_base_url = replace(base_url, r"/v1/?$" => "")
@@ -192,7 +197,7 @@ function override_providers!(base_url::String, api_key_env_var::String;
         anthropic_base_url,
         "Bearer",
         api_key_env_var,
-        Dict{String,String}(),
+        Dict{String,String}("User-Agent" => CLAUDE_CLI_USER_AGENT),
         cli_proxy_model_transform,
         AnthropicSchema(),
         "anthropic (overridden to cli_proxy_api)"