feat(proxy): add scoped proxy configuration and docs runbooks

- add scope-aware proxy schema and runtime wiring for providers/channels/tools

- add agent callable proxy_config tool for fast proxy setup

- standardize docs system with index, template, and playbooks

src/tools/browser.rs

@@ -736,7 +736,7 @@ impl BrowserTool {
             }
         });
 
-        let client = reqwest::Client::new();
+        let client = crate::config::build_runtime_proxy_client("tool.browser");
         let mut request = client
             .post(endpoint)
             .timeout(Duration::from_millis(self.computer_use.timeout_ms))