fix(web): correct the preset-layer contracts review found stale

None of these change behavior; each said something that was not true.

`SessionCwdConflict`'s doc block had been left stranded above the
`AgentPresetConflict` inserted under it, so one class carried a comment
about the other and the second carried none.

The roster comment named a `.system` directory that does not exist; the
shipped root is `config/agent-presets/`, and `system` is the trust its
entries carry.

The real-composition test attributed the disabled `api-gateway` row to
"side effects outside this process" alongside the port and the exporter.
It is disabled for a different reason — the api-proxy cannot mount in
this layer at all — and hiding that behind the same phrase would leave a
later layer unable to tell whether the line can come out.

One test claimed to refuse an adoption while asserting only that the
header records the preset; it now says what it checks.

`PERSONA_SECTION`/`PERSONA_ORDER` existed twice, once in the registry
that declares the slot and once restated in the row that replaces it —
a drift that would land a preset's persona beside the deployment's
instead of shadowing it. The registry exports them now.

The preset conflict message read "already runs agent preset undefined"
for a session that records none, which is the shape a deployment with no
roster produces; it names that case instead, with the regression that
reaches it through the gateway.

Finally, `PresetTree.write()` drops the `loader/config-update` the
inherited method emits — recorded on the override, since a future
edit-while-running flow needs its own persistence path.

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

@@ -642,7 +642,6 @@ class SubagentSessionOwnership extends Error {
   }
 }
 
-/** Requested identity already belongs to a session with another project cwd. */
 /**
  * The requested preset differs from the one this session already runs.
  *
@@ -658,12 +657,16 @@ class AgentPresetConflict extends Error {
     readonly existingPreset: string | undefined,
   ) {
     super(
-      `session "${sessionId}" already runs agent preset ${JSON.stringify(existingPreset)}; `
+      existingPreset === undefined
+        ? `session "${sessionId}" records no agent preset, so it cannot be adopted under one; `
+        + 'a deployment composing no roster records none on any session — '
+        : `session "${sessionId}" already runs agent preset ${JSON.stringify(existingPreset)}; `
       + `requested ${JSON.stringify(requestedPreset)}. A session's preset is fixed at creation.`,
     )
   }
 }
 
+/** Requested identity already belongs to a session with another project cwd. */
 class SessionCwdConflict extends Error {
   constructor(
     readonly sessionId: SessionId,