feat(web): draft a custom preset in a creator-mode session

The settings section gains the conversational authoring entry beside
copying: a dashed add-card (the Models page's affordance) that stages the
self-referential cordis preset and starts a new session on it, closing the
settings panel through the shell's new section owner-prop `close` — the
one shell affordance a section receives, for flows that leave settings.

The seat learns the difference between picking and staging: `select()`'s
immediate apply meets the still-current running session and drops the
stage as unservable, so the entry uses `stage()` and leaves the apply to
the list-change applier that fires when the started session becomes
current. `load()` stops regressing the display when its reply lands after
an applied stage was consumed — staged pick first, then the composition
the current session already carries, then the deployment default.

The authoring lane connects a workspace and drives the gesture to a
composed host session; the section goldens gain the entry.

packages/client/ui-settings/src/client/SettingsRoot.tsx

@@ -87,7 +87,7 @@ function SettingsPanel({ rows, renderSlot, activeId, onSelect, onClose }: PanelP
             </button>
           </div>
           <div className={css.options}>
-            {active !== undefined && renderSlot('settings.section', {}, { only: active })}
+            {active !== undefined && renderSlot('settings.section', { close: onClose }, { only: active })}
           </div>
         </div>
       </div>

packages/client/ui-settings/src/client/contract/slots.ts

@@ -83,12 +83,14 @@ export interface SettingsHeaderOwnerProps {
 
 /**
  * Owner share of a settings section entry. The shell owns modal visibility
- * and navigation; sections receive nothing but the render site (their data
- * arrives through their own inject faces and stores).
+ * and navigation; a section's data arrives through its own inject faces and
+ * stores. `close` is the one shell affordance a section receives, for flows
+ * that leave settings altogether (starting a session from a section) — the
+ * onboarding coordinator's `openSection`/`complete` precedent, inverted.
  */
 export interface SettingsSectionOwnerProps {
-  /** Marker field: section owner props are intentionally empty for now. */
-  children?: never
+  /** Close the settings panel (the shell owns the open state). */
+  close: () => void
 }
 
 /** Owner share of the currently active settings-backed onboarding step. */