fix(gen-skill-docs): throw when a template contains {{PREAMBLE}} twice

Hardens the #2508/#2362 class: a second {{PREAMBLE}} occurrence — even a
prose mention, which is exactly how spec/SKILL.md.tmpl re-expanded the full
~12K-token preamble mid-document — now fails generation with the template
path instead of silently shipping a doubled preamble. Pure exported guard
(assertSinglePreamble) called from resolvePlaceholders, unit-tested with the
original prose-mention shape.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/gen-skill-docs.test.ts

@@ -1,4 +1,5 @@
 import { describe, test, expect, beforeAll } from 'bun:test';
+import { assertSinglePreamble } from '../scripts/gen-skill-docs';
 import { COMMAND_DESCRIPTIONS } from '../browse/src/commands';
 import { SNAPSHOT_FLAGS } from '../browse/src/snapshot';
 import * as fs from 'fs';
@@ -1507,6 +1508,25 @@ describe('CHANGELOG_WORKFLOW resolver', () => {
   });
 });
 
+// --- Duplicate {{PREAMBLE}} guard (#2508/#2362) ---
+
+describe('assertSinglePreamble', () => {
+  test('one {{PREAMBLE}} passes', () => {
+    expect(() => assertSinglePreamble('a\n{{PREAMBLE}}\nb', 'x/SKILL.md.tmpl')).not.toThrow();
+  });
+
+  test('zero {{PREAMBLE}} passes (sections have none)', () => {
+    expect(() => assertSinglePreamble('no macro here', 'x/sections/y.md.tmpl')).not.toThrow();
+  });
+
+  test('a second occurrence throws with the template path — even in prose', () => {
+    // The original #2508 bug WAS a prose mention: "emitted by {{PREAMBLE}}'s
+    // preamble bash". Resolution is context-blind, so the guard must be too.
+    const tmpl = '{{PREAMBLE}}\n\n...later: emitted by {{PREAMBLE}}\'s preamble bash';
+    expect(() => assertSinglePreamble(tmpl, 'spec/SKILL.md.tmpl')).toThrow(/spec\/SKILL\.md\.tmpl.*2 times/);
+  });
+});
+
 // --- Parameterized resolver infrastructure tests ---
 
 describe('parameterized resolver support', () => {