feat(brief): locked sections, list formatting, auto/manual placeholder chips

- brief.machine: reducer refuses edits to locked (predefined) sections as
  defense-in-depth; LetterSection gains a `locked` flag
- rich-text: paragraphs gain optional `list` kind; editor gets bullet/numbered
  list buttons, keyboard shortcuts, and backspace-deletes-adjacent-chip
- placeholder chips distinguish auto-resolvable (grey) vs manual (yellow), in
  both the editor and the read-only preview
- fix: preview chip now renders matching {…} braces (was a one-sided ⌗ glyph),
  aligned with the editor's chip styling

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/brief/application/brief.store.ts

@@ -21,6 +21,8 @@ export class BriefStore {
   readonly role: Role = currentRole();
   readonly busy = signal(false);
   readonly lastError = signal<string | null>(null);
+  /** Surfaced autosave state for the indicator + aria-live region. */
+  readonly saveState = signal<'idle' | 'saving' | 'saved' | 'error'>('idle');
 
   private brief = computed<Brief | null>(() => {
     const s = this.model();
@@ -63,8 +65,26 @@ export class BriefStore {
   private async flushSave() {
     const b = this.brief();
     if (!b) return;
+    this.saveState.set('saving');
     const r = await this.adapter.save(b.sections);
-    if (!r.ok) this.lastError.set(r.error);
+    if (r.ok) {
+      this.saveState.set('saved');
+    } else {
+      this.lastError.set(r.error);
+      this.saveState.set('error');
+    }
+  }
+
+  /** Demo "start over": recreate the brief server-side and load the fresh view. */
+  async resetDemo() {
+    this.busy.set(true);
+    this.lastError.set(null);
+    clearTimeout(this.saveTimer);
+    const r = await this.adapter.reset();
+    this.busy.set(false);
+    this.saveState.set('idle');
+    if (r.ok) this.store.dispatch({ tag: 'BriefLoaded', brief: r.value.brief, availablePassages: r.value.availablePassages });
+    else this.lastError.set(r.error);
   }
 
   submit = () => this.transition(() => this.adapter.submit());