Add accessibility audit command (WCAG / platform guidelines)

[thymikee]

The snapshot gives the raw accessibility tree, which is useful. There is no structured audit that flags:

• Missing or insufficient labels
• Touch target size issues
• Missing hints
• Contrast issues

An audit accessibility (or similar) command that reports violations against WCAG or platform guidelines would let an agent build accessible apps by default.

Proposal: Add a command (e.g. audit accessibility or accessibility audit) that:

• Takes the current snapshot (or a snapshot scope).
• Runs a set of checks: missing labels, small touch targets, low contrast, missing accessibility hints, etc.
• Outputs a list of issues with element refs and severity/remediation hints (and optionally JSON for tooling).

Implementation could be in-process (analyze snapshot + screenshot for contrast) or delegate to platform tools (e.g. XCTest accessibility audits, Android Accessibility Scanner) where available.

Acceptance criteria:

• Command runs a defined set of accessibility checks on current UI
• Output includes element identification (ref or selector) and short description
• At least: missing labels, touch target size, contrast (if feasible)
• Docs and skill updated