test(agent-presets): name the rosterless case for what now makes it one

Empty `roots` stopped meaning "no roster" when the harness-home root became
a default, so the case that keeps the unjoined-agent diagnostic silent is the
one opting out of every root — which is what it already passes and what its
name now says.

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

@@ -528,10 +528,12 @@ describe('replacing a composition', () => {
     expect(warnings).toEqual([])
   })
 
-  it('says nothing when the deployment configures no roster at all', async () => {
+  it('says nothing when the composition opts out of every root', async () => {
     // Presets are optional: every surface except the Web bundle keeps its
     // model-facing rows in the host plane, so an agent with a chain of one is
-    // exactly right there and the diagnostic must stay silent.
+    // exactly right there and the diagnostic must stay silent. Opting out is
+    // what makes this rosterless — empty `roots` alone would still derive the
+    // harness-home root, which is a roster like any other.
     const rosterless = await harness({ default: 'standard', roots: [], includeUserRoot: false })
     const warnings: string[] = []
     rosterless.logger.warn = ((message: unknown) => { warnings.push(String(message)) }) as typeof rosterless.logger.warn