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.

apps/cli/tests/web-agent-presets.spec.ts

@@ -154,6 +154,36 @@ describe('the shipped Web composition', () => {
   })
 })
 
+describe('a forked session', () => {
+  it('inherits the composition its seeded history was produced under', async () => {
+    const parent = await ctx.agents.create({
+      sessionId: SessionId('preset-fork-parent'),
+      meta: { agentPreset: 'core-web' },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'core-web').then(() => undefined),
+    })
+    const inherited = parent.agent.session.header.agentPreset
+    const child = await ctx.agents.create({
+      sessionId: SessionId('preset-fork-child'),
+      meta: {
+        parentSession: SessionId('preset-fork-parent'),
+        seedLength: 0,
+        ...inherited === undefined ? {} : { agentPreset: inherited },
+      },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, inherited).then(() => undefined),
+    })
+    try {
+      // Composing nothing would leave the child empty: this layer moved every
+      // model-facing row out of the host plane, so there is nothing to inherit
+      // for free any more.
+      expect(toolNames(ctx, child.agent)).toEqual(toolNames(ctx, parent.agent))
+      expect(toolNames(ctx, child.agent).length).toBeGreaterThan(0)
+    } finally {
+      await child.dispose()
+      await parent.dispose()
+    }
+  })
+})
+
 describe('a session keeps the preset it was created with', () => {
   it('refuses to adopt a live session under a different preset', async () => {
     const handle = await ctx.agents.create({