fix(connection): fence every /api request behind one browser-trust check

The only browser-trust guard covered host.pickDirectory, while the
consequential methods (session.prompt drives bash) accepted any Host —
open to DNS rebinding, where a rebound page reads and writes the API as
if same-origin and only the Host header betrays the attacker's domain.

The pickDirectory-specific loopback guard becomes a prefix-wide fence:
Host must be loopback or an exact host[:port] from the new trustedHosts
config, an attached Origin must equal that authority, and explicit
cross-site markers are refused; requests without browser markers (curl,
tests, native clients) pass, because without a browser there is no
confused deputy. The loopback-socket check is dropped — binding policy
expresses reachability, and the fence is not an auth layer. The Agent
Note records the full threat model and the alternatives.

docs/config-catalog.md

@@ -270,6 +270,25 @@ Depends on: [`agentCore`](../packages/examples/agent-spine-demo/src/index.ts) ·
 
 Source: [`packages/examples/cli-demo/src/index.ts:26`](../packages/examples/cli-demo/src/index.ts)
 
+## `@deepseek-ai/dsh-client-connection`
+
+Requires: `httpServer` · `apiProxy`
+
+```ts config-catalog
+/** Plugin config: the deployment's non-loopback serving authorities. */
+export interface ConnectionConfig {
+  /**
+   * Exact `host[:port]` authorities this deployment serves beyond loopback.
+   * The /api trust fence refuses any request whose Host is neither loopback
+   * nor listed here, so a non-loopback (`0.0.0.0`) deployment must declare
+   * the names it is reached by.
+   */
+  trustedHosts?: string[]
+}
+```
+
+Source: [`packages/client/connection/src/index.ts:20`](../packages/client/connection/src/index.ts)
+
 ## `@deepseek-ai/dsh-client-hmr`
 
 Requires: `clientModuleHost` · `httpServer`
@@ -2143,7 +2162,6 @@ Source: [`packages/context/workspace-context/src/config.ts:17`](../packages/cont
 These load from a `cordis.yml` entry with no `config:` block; they declare no config surface.
 
 - `@deepseek-ai/dsh-agent` ([`packages/core/agent/src/index.ts`](../packages/core/agent/src/index.ts))
-- `@deepseek-ai/dsh-client-connection` — requires `httpServer` · `apiProxy` ([`packages/client/connection/src/index.ts`](../packages/client/connection/src/index.ts))
 - `@deepseek-ai/dsh-client-locale` ([`packages/client/locale/src/index.ts`](../packages/client/locale/src/index.ts))
 - `@deepseek-ai/dsh-client-modules` — requires `httpServer` · `loader` ([`packages/client/modules/src/index.ts`](../packages/client/modules/src/index.ts))
 - `@deepseek-ai/dsh-client-runtime` ([`packages/client/runtime/src/index.ts`](../packages/client/runtime/src/index.ts))