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/tunnel/custom.rs

@@ -123,7 +123,7 @@ impl Tunnel for CustomTunnel {
     async fn health_check(&self) -> bool {
         // If a health URL is configured, try to reach it
         if let Some(ref url) = self.health_url {
-            return reqwest::Client::new()
+            return crate::config::build_runtime_proxy_client("tunnel.custom")
                 .get(url)
                 .timeout(std::time::Duration::from_secs(5))
                 .send()