docs: README describes the phone layout that shipped in 3.1

[m-vawter]

The README's "What DOMinic is" bullet still says phone viewports are not supported yet, and that the phone layout is roadmap step 3.1. Step 3.1 shipped in #100, so the bullet now describes what main does:

built for desktop browsers, with a phone layout below 768px: windows stack as full-width cards and the taskbar becomes a scrollable bottom sheet (ADR 0004; roadmap step 3.1).

The wording follows #99's "Done when" boxes.

Closes #121.

Verification

• markdownlint-cli2 0.18.1 with the repo's .markdownlint.jsonc: 0 errors.
• No other README line mentions phones or mobile. docs/JUDGES.md also lists "mobile degradation of the shell" as deferred. That page is the hackathon tour, so this PR leaves it alone.

🤖 Generated with Claude Code

[xcjs]

Verified against main: README still claims phone viewports unsupported (step 3.1 as design-target), but #100 shipped the $< layout. New wording matches the shipped behavior (stacked full-width cards, scrollable bottom sheet) and #99's Done-when boxes. Closes #121 correctly.