feat: add websocket browser service runtime

Wire the service/browser runtime onto the websocket-driven execution path and add the new browser/service modules needed for the submit flow and runtime integration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/browser/bridge_transport.rs

@@ -0,0 +1,9 @@
+use crate::browser::bridge_contract::{BridgeBrowserActionReply, BridgeBrowserActionRequest};
+use crate::pipe::PipeError;
+
+pub trait BridgeActionTransport: Send + Sync {
+    fn execute(
+        &self,
+        request: BridgeBrowserActionRequest,
+    ) -> Result<BridgeBrowserActionReply, PipeError>;
+}