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/core/system-prompt/src/index.ts

@@ -110,6 +110,17 @@ export interface PromptAssembly {
   variables: Record<string, string | undefined>
 }
 
+/**
+ * The deployment persona's section name and order. Exported because a
+ * composition can replace this slot — an agent preset shadows the
+ * deployment's persona with its own — and both sides naming the same section
+ * is what makes the replacement work rather than duplicate.
+ */
+export const PERSONA_SECTION = 'deployment:persona'
+
+/** Prompt order of the persona slot; the first section a model reads. */
+export const PERSONA_ORDER = 0
+
 /** Valid variable names: how they are written between the braces. */
 const VARIABLE_NAME = /^[a-z][a-z0-9_]*$/
 
@@ -337,8 +348,8 @@ export class SystemPrompt extends Service {
       })
     }
     this.section({
-      name: 'deployment:persona',
-      order: 0,
+      name: PERSONA_SECTION,
+      order: PERSONA_ORDER,
       // The fallback narrows the optional input type; the schema already defaults it.
       text: config.persona ?? '',
     })