fix(web): compose a forked session, and give the shell realm its provider

Two consequences of moving the agent plane behind presets, both invisible
until the host plane stopped carrying model-facing rows.

`sessions.fork` built its child with a bare `installTarget` and a `meta`
without `agentPreset`. That was harmless while every tool sat in the host
plane — the child inherited them for free. It now comes up with an EMPTY
tool set. The child composes the parent's preset instead, for the same
reason a resumed session keeps its own: the seeded history was produced
under those tools.

`bashEnv` lives in its own `dsh-bash-env` row rather than inside
`tool-bash`, so a preset that isolates the realm must compose the provider
beside its consumer; the host row is disabled here like every other
model-facing one. Nothing outside the agent plane injects `bashEnv`, so it
stays per-session.

packages/bundle/web-app/cordis.patch.yml

@@ -201,6 +201,9 @@
 # absent from a surface overlay would silently reappear the day someone reorders
 # the composition.
 
+- id: bash-env
+  disabled: true
+
 - id: tool-bash
   disabled: true