fix(web): default to loopback binding

.agents/notes/implemented/feature/2026-07-22-web-bind-address.md

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+# Agent Note: Explicit web bind address
+
+Status: implemented
+
+English | [中文](2026-07-22-web-bind-address.zh.md)
+
+## Problem
+
+`dsh web` binds every network interface even when its browser runs on the same machine. Local use therefore exposes an unauthenticated development server without an explicit operator choice, while remote-container and LAN-browser use still needs a supported way to accept non-loopback connections.
+
+The HTTP carrier also hides the bind address inside `startWebServer()`, so alternate shells cannot state their own network policy at the package boundary.
+
+## Decision
+
+`dsh web` binds `127.0.0.1` by default. The CLI accepts `--host 0.0.0.0` as the explicit all-interface mode and rejects other values so its network modes remain a small, deliberate contract. All-interface mode keeps printing the loopback URL and, when available, the first external IPv4 URL.
+
+`WebServerOptions.host` is required. The HTTP carrier passes that value to `node:http` without supplying a fallback, leaving each shell responsible for its bind policy. Programmatic carrier consumers may select another hostname or address directly.
+
+## Alternatives considered
+
+**Keep `0.0.0.0` as the default.** Rejected because ordinary same-machine use does not need network-wide reachability and should not acquire it implicitly.
+
+**Use a boolean exposure flag.** Rejected because `--host 0.0.0.0` names the resulting socket behavior directly and matches the underlying server option without introducing a second term.
+
+**Default inside `startWebServer()`.** Rejected because the carrier has multiple possible shells and no basis for choosing their deployment policy. Requiring `host` makes the choice visible at every assembly call.
+
+## Consequences
+
+Local `dsh web` starts remain reachable at `http://127.0.0.1:3080`; a browser on another machine must opt in with `dsh web --host 0.0.0.0`. The CLI does not yet expose custom interface addresses or IPv6 modes, while programmatic carrier consumers retain that flexibility. Server tests pin both loopback and all-interface forwarding into the Node listen boundary, and the web smoke continues to exercise the default CLI path.