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/host/apiproxy/src/api-proxy.ts

@@ -1898,6 +1898,12 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
           })
         }
         const childId = `session-${randomUUID()}` as SessionId
+        // The child inherits the parent's composition for the same reason a
+        // resumed session keeps its own: the seeded history was produced under
+        // those tools, and composing anything else would strand the tool calls
+        // it already carries. Now that no model-facing row sits in the host
+        // plane, composing nothing would leave the child with no tools at all.
+        const forkComposition = await composeAgent(source.header.agentPreset)
         try {
           await ctx.agents.create({
             sessionId: childId,
@@ -1906,9 +1912,12 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
               ...source.header.cwd === undefined ? {} : { cwd: source.header.cwd },
               parentSession: source.id,
               seedLength: cut,
+              ...forkComposition.agentPreset === undefined
+                ? {}
+                : { agentPreset: forkComposition.agentPreset },
             },
             agentOptions,
-            setup: installTarget,
+            setup: forkComposition.setup,
           })
         } catch (error: unknown) {
           return err(request, {