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).

tsconfig.host.json

@@ -38,6 +38,7 @@
     "apps/web/tests/access-confirmation.e2e.ts",
     "apps/web/tests/shipped-composition.e2e.ts",
     "apps/web/tests/startup-auto-selection.e2e.ts",
+    "apps/web/tests/workspace-file-open.e2e.ts",
     "apps/cli/tests/**/*.ts",
     "examples/*/src/**/*.ts",
     "examples/*/start.ts",