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/helpers/capture-parity-baseline.ts

@@ -57,7 +57,7 @@ export interface CaptureOptions {
 }
 
 /** Extract the frontmatter description from a SKILL.md file. Empty string if none. */
-function extractDescription(content: string): string {
+export function extractDescription(content: string): string {
   if (!content.startsWith('---\n')) return '';
   const fmEnd = content.indexOf('\n---', 4);
   if (fmEnd === -1) return '';