feat(atomic-a11y): add AI WCAG audit orchestrator and prompts

[y-lakhdar]

Context

Split from #7125 for reviewability. This is PR 5c — the main orchestrator that ties together the browser capture and LLM modules from 5b.

What this PR does

AI WCAG audit orchestrator (ai-wcag-audit.ts — 991 lines):

• CLI argument parsing (component filter, concurrency, dry-run, resume)
• Storybook story discovery and component matching
• 4-call LLM evaluation pipeline splitting WCAG criteria across batched calls
• Concurrency pool for parallel component evaluation
• Progress tracking with resume capability (restart from where it stopped)
• Delta file output for integration with the manual audit workflow (5a)
• Structured JSON report generation

LLM prompts (prompts.ts — 730 lines):

• Prompt construction for 4 evaluation calls
• WCAG criteria grouped by domain (perceivable, operable, understandable, robust)
• Screenshot and a11y tree context injection
• Structured output format specification for reliable JSON parsing

Entry point:

• scripts/ai-wcag-audit.mjs — ESM wrapper

Try it

git checkout feat/a11y-audit-orchestrator && pnpm install
cd packages/atomic-a11y && pnpm build

# Dry run (requires running Storybook)
node scripts/ai-wcag-audit.mjs --dry-run

# Audit a single component (requires OPENAI_API_KEY + Storybook)
node scripts/ai-wcag-audit.mjs --component atomic-product-link

# Resume a previous run
node scripts/ai-wcag-audit.mjs --resume

PR Chain (5c of 7)

#	PR	Branch	Description
1	#7111	feat/a11y-package-scaffold	Package scaffolding
2	#7122	feat/a11y-shared-foundation	Shared types, constants, utilities
3	#7123	feat/a11y-reporter	VitestA11yReporter
4	#7124	feat/a11y-openacr	OpenACR report generator
5a	#7187	feat/a11y-delta-workflow	Shared foundation + delta workflow + issue generator
5b	#7188	feat/a11y-browser-llm	Browser capture + LLM client modules
5c	#7189	feat/a11y-audit-orchestrator	AI WCAG audit orchestrator + prompts ← this PR
6	#7126	feat/a11y-reporter-wiring	Wire reporter into atomic vitest config
7	#7117	feat/a11y-ci-integration	Weekly a11y scan workflow

Replaces #7125 (split into 5a/5b/5c)

KIT-5471