feat(web): open a produced file from the conversation

Serve one file at a time out of a Session's workspace under /f on the web
transport, and point the conversation's existing file-open affordance at it.
Clicking a write/edit/read row's path now opens that file in a browser tab —
including from a LAN client, where the Host's system opener is fenced to
loopback and answered nothing.

- /f/<sessionId>/<segments> in client-connection, behind the same
  browser-trust fence as /api; realpath confinement, streamed reads,
  GET/HEAD only, nosniff + no-store.
- Script-capable documents carry CSP sandbox: model-authored markup must not
  be same-origin with /api, where events.mux is a readable GET stream.
- ApiProxy.workspaceRootOf answers where a Session's files live without
  resuming an agent; the client program cannot reach the core services.
- The /f URL shape lives in dsh-host-apiproxy/api so both ends share one
  encoding (client bundles may not value-import another plugin).

packages/client/runtime/src/client/contract/workspaces.ts

@@ -56,6 +56,17 @@ export interface IWorkspaces {
    * @param path - absolute or host-resolvable path.
    */
   openPath(path: string): Promise<void>
+  /**
+   * URL serving one file out of a session's workspace, for a UI that opens a
+   * produced file in the browser instead of on the Host machine.
+   * @param sessionId - the session whose cwd anchors the path.
+   * @param cwd - that session's working directory, or `undefined` when unknown.
+   * @param path - the path a tool reported (absolute, or relative to `cwd`).
+   * @returns the origin-relative URL, or `undefined` when the path lies
+   * outside the workspace — which this transport never serves, leaving
+   * {@link IWorkspaces.openPath} as the only way to reach it.
+   */
+  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined
   /**
    * Rename a Workspace.
    * @param workspaceId - target workspace.

packages/client/runtime/src/client/workspaces/service.ts

@@ -5,6 +5,7 @@ import type {
   DirectoryListing, IApiClient, RpcError,
   SessionId, WorkspaceId, WorkspaceView,
 } from '@deepseek-ai/dsh-client-connection/client'
+import { workspaceFileSegments, workspaceFileUrl } from '@deepseek-ai/dsh-host-apiproxy/api'
 import type { SnapshotStore } from '../contract/store.ts'
 import { createSnapshotStore } from '../contract/store.ts'
 import type { SessionsPort, SessionsPortList } from '../contract/sessions-port.ts'
@@ -239,6 +240,19 @@ export class WorkspacesService implements IWorkspaces {
     }
   }
 
+  /**
+   * URL serving one file out of a session's workspace.
+   * @param sessionId - the session whose cwd anchors the path.
+   * @param cwd - that session's working directory, or `undefined` when unknown.
+   * @param path - the path a tool reported (absolute, or relative to `cwd`).
+   * @returns the origin-relative URL, or `undefined` for a path outside the workspace.
+   */
+  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined {
+    const segments = workspaceFileSegments(cwd, path)
+    if (segments === undefined) return undefined
+    return workspaceFileUrl(sessionId, segments)
+  }
+
   /**
    * Rename a Workspace.
    * @param workspaceId - target workspace.