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/domain/brief.ts

@@ -47,6 +47,9 @@ export interface LetterSection {
   readonly sectionKey: string;
   readonly title: string;
   readonly required: boolean;
+  // Predefined template sections (aanhef, slot) arrive locked and prefilled — the drafter
+  // composes only the unlocked section(s). The reducer refuses edits to locked sections.
+  readonly locked: boolean;
   readonly blocks: readonly LetterBlock[];
 }