feat(brief): besluit guidance + read-only notice (role clarity)

Surface the besluit-driven assistance that was previously silent: a pure
besluitGuidance() (kern passage count + needs-reason flag) rendered as a hint below
the besluit panel in behandel-scherm (warning to pick a reden, else info on how many
standaardteksten were inserted). Add a read-only notice on letter-composer for a pure
viewer (no edit/approve/reject/send right, e.g. admin) so the read-only letter isn't
mistaken for a broken editor — the "reverted to a view of the letter" confusion was a
role issue (non-drafter sees the composer). +besluitGuidance spec.

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

src/app/brief/domain/besluit.spec.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect } from 'vitest';
 import { Besluit, LetterBlock, LibraryPassage } from './brief';
-import { inferSelection, passagesForBesluit, redenenFor } from './besluit';
+import { besluitGuidance, inferSelection, passagesForBesluit, redenenFor } from './besluit';
 
 const block = (t: string): LibraryPassage['content'] => ({
   paragraphs: [{ nodes: [{ type: 'text', text: t }] }],
@@ -117,3 +117,20 @@ describe('inferSelection', () => {
     expect(inferSelection(blocks, lib)).toEqual({ besluit: 'positief', reasons: [] });
   });
 });
+
+describe('besluitGuidance', () => {
+  it('positief: counts inserted passages, no reden needed (positief has no redenen)', () => {
+    expect(besluitGuidance(lib, 'positief', [])).toEqual({ insertedCount: 2, needsReason: false });
+  });
+
+  it('negatief without a reden: flags that a reden must be chosen', () => {
+    expect(besluitGuidance(lib, 'negatief', [])).toEqual({ insertedCount: 2, needsReason: true });
+  });
+
+  it('negatief with a reden: no longer flags, and the reason passage is counted', () => {
+    expect(besluitGuidance(lib, 'negatief', ['onvoldoende_scholing'])).toEqual({
+      insertedCount: 3,
+      needsReason: false,
+    });
+  });
+});