[codex] compact agent guidance

[aj47]

Summary

• Tightens the root AGENTS.md guidance from 73 lines to 39 lines.
• Adds the product/UI principle that every multiline UI surface should collapse to a one-line state.
• Consolidates existing repo gotchas without expanding the instruction file.

Validation

• git diff --check
• Docs-only change; app tests were not run.