fix(sources): update pdf-ua-1 URL in source registry and affected agents

[AdamMagued]

Description

Resolves #204.

Updates the source URL for pdf-ua-1 ("PDF/UA in a Nutshell") from https://pdfa.org/resource/pdfua-in-a-nutshell/ to the canonical active resource URL https://www.pdfa.org/resource/pdfua-in-a-nutshell/.

Also updates the corresponding citations for the affected agents (pdf-accessibility and document-accessibility-wizard) in skills/a11y-core/references/sources.md to reference https://www.pdfa.org/resource/pdfua-in-a-nutshell/ instead of the broken legacy link https://www.pdfa.org/pdfua/.

Refreshed lastVerified in docs/standards/source-registry.json to 2026-09-30.

Changes

• docs/standards/source-registry.json: Updated pdf-ua-1 URL to https://www.pdfa.org/resource/pdfua-in-a-nutshell/ and refreshed lastVerified to 2026-09-30.
• skills/a11y-core/references/sources.md: Updated pdf-accessibility and document-accessibility-wizard source links to https://www.pdfa.org/resource/pdfua-in-a-nutshell/.

Verification

• Verified the canonical URL on pdfa.org: https://www.pdfa.org/resource/pdfua-in-a-nutshell/.
• Full verification suite ran cleanly via npm run verify: 14 gates passed, 1 optional gate skipped (live client session due to absence of CLI on runner).
• No linting or conformance issues introduced.

Accessibility Checklist

Structure

• N/A (documentation and configuration change only)

Testing

• Tested with full test suite (npm run verify)
• No new accessibility linting warnings

Source Citations (Agent Changes Only)

• All factual claims include inline links to authoritative sources
• Sources section included at end of any new agent file or substantive guidance
• Source URLs verified as reachable and current
• New sources added to SOURCE_REGISTRY.json if they are primary references for an agent domain