Align repository guidance for coding agents

[mrmees]

Summary

• add a concise root AGENTS.md defining repository constraints, architecture boundaries, verification commands, and the documentation trust hierarchy
• convert root and UI CLAUDE.md files into short pointers and move UI-specific law into a scoped docs/ui_design/AGENTS.md
• add scripts/check.sh as the canonical local and CI verification entry point
• consolidate documentation hygiene in that script and remove duplicated workflow logic
• correct stale command-artifact guidance and remove authoritative references to ignored planning and sketch files

Why

Agents could automatically consume a large ignored CLAUDE.md and follow local historical material that was not present in public clones or guaranteed to be current. This gives the repository one tracked instruction source, clearly separates authoritative documentation from archaeology, and adds executable drift checks.

Impact

Future contributors and coding agents have a smaller, consistent orientation path and one command for normal verification. Application behavior is unchanged.

Validation

• shellcheck scripts/check.sh
• ./scripts/check.sh --docs-only
• git diff --check
• ./scripts/check.sh
  • host unit tests
  • Android lint
  • debug APK assembly

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