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/shared/ui/placeholder-chip/placeholder-chip.component.ts

@@ -14,7 +14,9 @@ import { Component, computed, input } from '@angular/core';
       border-radius:var(--rhc-border-radius-sm);
       padding:0 0.35em;line-height:1.6;border:1px solid transparent;white-space:nowrap;
     }
-    .chip::before{content:'⌗';opacity:0.6;font-weight:700}
+    /* Braces use unicode escapes; a literal { in a CSS content string breaks the style parser. */
+    .chip::before{content:'\\7B';opacity:0.6;font-weight:700}
+    .chip::after{content:'\\7D';opacity:0.6;font-weight:700}
     .chip--auto{background:var(--rhc-color-cool-grey-100);color:var(--rhc-color-foreground-default)}
     .chip--manual{background:var(--rhc-color-geel-100);color:var(--rhc-color-foreground-default)}
     .chip--warning{background:var(--rhc-color-geel-100);border-color:var(--rhc-color-border-default);color:var(--rhc-color-foreground-default)}