test: catalog ratchet reads committed content; SDK unit pins follow D1a default

Two follow-ups from the verification runs:

1. skill-size-budget's catalog estimate still flaked under --parallel
   (8356, then 8041, vs 4177 solo) even after filtering to tracked
   skills: sibling workers REGENERATE real SKILL.md files mid-run, so
   any live-tree read is a moving target. The ratchet now reads each
   tracked skill's frontmatter from git show HEAD: — the catalog that
   ships — which no concurrent worker can perturb.

2. agent-sdk-runner unit pins asserted the old Opus default through the
   default-flow fixtures; flipped to the Sonnet default (the explicit-
   override pass-through pins keep Opus — that path is unchanged).

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

test/agent-sdk-runner.test.ts

@@ -45,7 +45,7 @@ function uuid(): string {
   return `00000000-0000-0000-0000-${String(++uuidCounter).padStart(12, '0')}`;
 }
 
-function systemInit(model = 'claude-opus-4-7', version = '2.1.117'): SDKMessage {
+function systemInit(model = 'claude-sonnet-4-6', version = '2.1.117'): SDKMessage {
   return {
     type: 'system',
     subtype: 'init',
@@ -77,7 +77,7 @@ function assistantTurn(
       id: 'msg_' + uuid(),
       type: 'message',
       role: 'assistant',
-      model: 'claude-opus-4-7',
+      model: 'claude-sonnet-4-6',
       content: blocks.map((b) => ({ ...b })),
       stop_reason: 'end_turn',
       stop_sequence: null,
@@ -259,7 +259,7 @@ describe('runAgentSdkTest — happy path', () => {
     expect(result.turnsUsed).toBe(2);
     expect(result.costUsd).toBe(0.05);
     expect(result.sdkClaudeCodeVersion).toBe('2.1.117');
-    expect(result.model).toBe('claude-opus-4-7');
+    expect(result.model).toBe('claude-sonnet-4-6');
     expect(result.firstResponseMs).toBeGreaterThanOrEqual(0);
   });
 
@@ -699,7 +699,7 @@ describe('toSkillTestResult', () => {
     expect(s.output).toBe('hi');
     expect(s.costEstimate.estimatedCost).toBe(0.02);
     expect(s.costEstimate.turnsUsed).toBe(1);
-    expect(s.model).toBe('claude-opus-4-7');
+    expect(s.model).toBe('claude-sonnet-4-6');
     expect(s.firstResponseMs).toBeNumber();
     expect(s.maxInterTurnMs).toBeNumber();
     expect(s.transcript).toBeArray();
@@ -715,7 +715,7 @@ describe('validateFixtures', () => {
     return {
       id: 'test-fixture',
       overlayPath: 'model-overlays/opus-4-7.md',
-      model: 'claude-opus-4-7',
+      model: 'claude-sonnet-4-6',
       trials: 10,
       setupWorkspace: () => {},
       userPrompt: 'go',