Document the next quality milestone

[mcvalosborne]

What changed

• adds an ordered next-quality milestone to the README
• defines measurable exit criteria for visual QA, playtesting, accessibility, content depth, campaign continuity, project tracking, engagement measurement, and community contributions
• adds corresponding execution and evidence requirements to AGENTS.md
• corrects the outdated README statement that described the shipped mobile action control as pending

Why

The three-level campaign is functional, but the remaining path to a fully validated game needs to be explicit for future contributors and coding agents.

Verification

• npm run verify (34 tests passing)
• git diff --check