Add axe-core and Pa11y accessibility testing tools

Add two widely-used accessibility analysis tools for HTML:

- axe-core (15K+ GitHub stars, MPL-2.0): The industry standard for
  automated accessibility testing, used by Google Lighthouse and Microsoft
  Accessibility Insights. Tests against WCAG 2.0/2.1/2.2 guidelines.

- Pa11y (4.5K+ GitHub stars, LGPL-3.0): Command-line accessibility
  testing tool with CI/CD integration support and multiple reporters.

Both tools perform static analysis of HTML to identify accessibility
violations (missing alt text, insufficient color contrast, missing ARIA
attributes, heading hierarchy issues, etc.).

Author: shibley

data/tools/axe-core.yml

@@ -0,0 +1,14 @@
+name: axe-core
+categories:
+  - linter
+tags:
+  - html
+license: MPL-2.0
+types:
+  - cli
+source: 'https://github.com/dequelabs/axe-core'
+homepage: 'https://www.deque.com/axe/'
+description: >-
+  Accessibility engine for automated Web UI testing. Tests HTML against
+  WCAG 2.0, 2.1, and 2.2 guidelines. Used by Google Lighthouse, Microsoft
+  Accessibility Insights, and thousands of organizations worldwide.

data/tools/pa11y.yml

@@ -0,0 +1,14 @@
+name: Pa11y
+categories:
+  - linter
+tags:
+  - html
+license: LGPL-3.0
+types:
+  - cli
+source: 'https://github.com/pa11y/pa11y'
+homepage: 'https://pa11y.org/'
+description: >-
+  Automated accessibility testing tool that runs HTML CodeSniffer or axe-core
+  from the command line. Supports CI/CD integration, multiple reporters, and
+  testing against WCAG 2.1 AA standards.