Project instructions: CLAUDE.md, AGENTS.md, rules files

Learn to give the agent a permanent 'project brief' it reads every time: how to run, test and lint the project, folder structure, coding style, do's and don'ts. Claude Code reads CLAUDE.md (create it with /init); Codex and many tools read AGENTS.md; Cursor uses .cursor/rules; Copilot uses .github/copilot-instructions.md. Keep it short and update it when the agent repeats a mistake. Use a PLAN.md for multi-step work.