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.machine.ts

@@ -63,6 +63,17 @@ function mapSection(brief: Brief, sectionKey: string, f: (s: LetterSection) => L
   return { ...brief, sections: brief.sections.map((s) => (s.sectionKey === sectionKey ? f(s) : s)) };
 }
 
+/** The section a block currently lives in, or undefined if the block is gone. */
+function sectionKeyOfBlock(brief: Brief, blockId: string): string | undefined {
+  return brief.sections.find((s) => s.blocks.some((b) => b.blockId === blockId))?.sectionKey;
+}
+
+/** A section accepts edits only when it is not a locked (predefined) template section. */
+function isSectionEditable(brief: Brief, sectionKey: string | undefined): boolean {
+  const section = brief.sections.find((s) => s.sectionKey === sectionKey);
+  return !!section && !section.locked;
+}
+
 function mapBlocks(brief: Brief, f: (blocks: readonly LetterBlock[]) => LetterBlock[]): Brief {
   return { ...brief, sections: brief.sections.map((s) => ({ ...s, blocks: f(s.blocks) })) };
 }
@@ -126,19 +137,29 @@ export function reduce(s: BriefState, m: BriefMsg): BriefState {
     case 'Seed':
       return m.state;
 
+    // Section-level guard (defense-in-depth): locked sections never accept edits, even if a
+    // Msg reaches the reducer. The UI already hides controls for locked sections.
     case 'PassagesInserted':
-      return withEdit(s, (b) => insertPassages(b, m.sectionKey, m.passages));
+      return withEdit(s, (b) => (isSectionEditable(b, m.sectionKey) ? insertPassages(b, m.sectionKey, m.passages) : b));
     case 'FreeTextBlockAdded':
-      return withEdit(s, (b) => addFreeText(b, m.sectionKey));
+      return withEdit(s, (b) => (isSectionEditable(b, m.sectionKey) ? addFreeText(b, m.sectionKey) : b));
     case 'BlockContentEdited':
-      return withEdit(s, (b) => editBlockContent(b, m.blockId, m.content));
+      return withEdit(s, (b) =>
+        isSectionEditable(b, sectionKeyOfBlock(b, m.blockId)) ? editBlockContent(b, m.blockId, m.content) : b,
+      );
     case 'BlockRemoved':
-      return withEdit(s, (b) => mapBlocks(b, (blocks) => blocks.filter((x) => x.blockId !== m.blockId)));
+      return withEdit(s, (b) =>
+        isSectionEditable(b, sectionKeyOfBlock(b, m.blockId))
+          ? mapBlocks(b, (blocks) => blocks.filter((x) => x.blockId !== m.blockId))
+          : b,
+      );
     case 'BlockMovedWithinSection':
       return withEdit(s, (b) =>
-        mapBlocks(b, (blocks) =>
-          blocks.some((x) => x.blockId === m.blockId) ? moveWithinSection(blocks, m.blockId, m.toIndex) : [...blocks],
-        ),
+        isSectionEditable(b, sectionKeyOfBlock(b, m.blockId))
+          ? mapBlocks(b, (blocks) =>
+              blocks.some((x) => x.blockId === m.blockId) ? moveWithinSection(blocks, m.blockId, m.toIndex) : [...blocks],
+            )
+          : b,
       );
 
     case 'Submitted':