fix(agent-presets): re-link recompose through the mount-time binding

The auto-merge kept recompose on the removed open re-link. It now moves
the binding this roster kept from the agent's mount; an agent that never
composed one has nothing to re-link, so the switch is its first bind —
exactly a mount — and once bound only the kept binding can move it.

packages/preset/agent-presets/src/index.ts

@@ -225,7 +225,10 @@ export class AgentPresets extends Service {
    * and permanent, so the old composition stays for its other agents and the
    * new one is ensured BEFORE the link moves. An unknown or unusable preset
    * therefore throws with the agent exactly as it was — there is no torn-down
-   * state to restore.
+   * state to restore. The re-link runs through the binding this roster kept
+   * from the agent's mount — dsh-scope's only re-link authority. An agent
+   * that never composed one has nothing to re-link: the switch is then the
+   * agent's first bind, exactly a mount.
    * @param agentCtx - the agent's scope context.
    * @param id - the preset to compose the agent from instead.
    * @returns the preset now installed.
@@ -238,7 +241,12 @@ export class AgentPresets extends Service {
     }
     const preset = await this.resolve(id)
     const standing = await this.ensureStanding(preset)
-    setScopeParent(agentKey, standing.key)
+    const binding = this.bindings.get(agentKey)
+    if (binding === undefined) {
+      this.bindings.set(agentKey, bindScopeParent(agentKey, standing.key))
+    } else {
+      binding.rebind(standing.key)
+    }
     return preset
   }
 

packages/preset/agent-presets/tests/mount.spec.ts

@@ -368,9 +368,19 @@ describe('replacing a composition', () => {
     expect(toolNames(ctx, handle.agent)).toEqual(['alpha'])
   })
 
-  it('leaves an agent that never composed one with nothing to restore', async () => {
+  it('binds an agent that never composed one as its first mount', async () => {
+    // No binding exists to re-link, so the switch is the agent's first bind
+    // — and once bound, only this roster's kept binding can move it again.
     const handle = await ctx.agents.create({ sessionId: SessionId('sess-bare') })
 
+    await ctx.agentPresets.recompose(handle.agent.ctx, 'standard')
+
+    expect(toolNames(ctx, handle.agent)).toEqual(['alpha'])
+  })
+
+  it('leaves an agent that never composed one with nothing to restore', async () => {
+    const handle = await ctx.agents.create({ sessionId: SessionId('sess-bare-broken') })
+
     await expect(ctx.agentPresets.recompose(handle.agent.ctx, 'broken'))
       .rejects.toThrow(/failed to mount/)