Add pixel-level visual screenshot diff (in addition to tree diff) #97

Screenshots exist via the screenshot command, and diff snapshot compares the accessibility tree structure. There is no pixel-level visual diff. When building UI, the question is often "does this look right?" not just "is the tree shaped right?" — visual regressions (wrong colors, alignment, cropping) can be missed by tree diffs.

Proposal: Add a diff screenshot (or similar) command that:

• Captures a screenshot and compares it to a baseline image (e.g. from previous run or a provided path).
• Uses perceptual hashing or pixel comparison; reports difference (e.g. score or pass/fail) and optionally saves the diff image.
• Integrates with session/baseline flow where it makes sense (e.g. first run saves baseline, subsequent runs compare).

Acceptance criteria:

• Command to compare current screenshot to a baseline (file or session-stored)
• Output includes at least pass/fail or a difference metric; optional diff image output
• Document baseline storage and CI usage
• Docs and skill updated