feat(model-overlays): add fable-5, opus-4-8, and sonnet-5 overlays + resolver mappings

model-overlays/ had no entry for the current Claude generation, so every
session on a Claude 5 family or Opus 4.8 model fell through to the generic
claude.md nudges. Adds the three overlays with resolver mappings and
per-overlay tests; generated output for the default host is unchanged
(overlays activate by detected model).

Closes #2509.

Contributed by @chrisquorum (PRs #2246, #2243, #2247).

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

test/model-overlay-sonnet-5.test.ts

@@ -0,0 +1,56 @@
+/**
+ * Sonnet 5 model overlay — gate-tier assertions on the family nudges.
+ *
+ * sonnet-5 inherits the claude base and adds Sonnet-5 family nudges: literal
+ * instruction following (state scope explicitly), scope work to the request
+ * (raise effort rather than prompting around shallow reasoning), and
+ * verbosity that tracks task complexity.
+ */
+import { describe, test, expect } from 'bun:test';
+import * as fs from 'fs';
+import * as path from 'path';
+import type { TemplateContext } from '../scripts/resolvers/types';
+import { HOST_PATHS } from '../scripts/resolvers/types';
+import { generateModelOverlay } from '../scripts/resolvers/model-overlay';
+
+function makeCtx(model: string): TemplateContext {
+  return {
+    skillName: 'test-skill',
+    tmplPath: 'test.tmpl',
+    host: 'claude',
+    paths: HOST_PATHS.claude,
+    preambleTier: 2,
+    model,
+  };
+}
+
+const ROOT = path.resolve(__dirname, '..');
+
+describe('Sonnet 5 overlay — family nudges', () => {
+  test('raw sonnet-5.md contains the literal-instructions nudge', () => {
+    const raw = fs.readFileSync(path.join(ROOT, 'model-overlays/sonnet-5.md'), 'utf-8');
+    expect(raw).toContain('Instructions are read literally');
+  });
+
+  test('resolved overlay inherits from claude base (INHERIT:claude)', () => {
+    const out = generateModelOverlay(makeCtx('sonnet-5'));
+    expect(out).toContain('Todo-list discipline');
+    expect(out).toContain('subordinate');
+  });
+
+  test('resolved overlay carries the Sonnet 5 nudges', () => {
+    const out = generateModelOverlay(makeCtx('sonnet-5'));
+    expect(out).toContain('Instructions are read literally');
+    expect(out).toContain('Scope work to the request');
+  });
+
+  test('resolved overlay has no unresolved INHERIT directive', () => {
+    const out = generateModelOverlay(makeCtx('sonnet-5'));
+    expect(out).not.toContain('{{INHERIT:');
+  });
+
+  test('claude overlay (base) does not carry the Sonnet 5 nudge', () => {
+    const out = generateModelOverlay(makeCtx('claude'));
+    expect(out).not.toContain('Instructions are read literally');
+  });
+});