Instruction Files and Scoped Context Assembly

A short text file kept in a repository can tell a coding agent how that project works: the commands to run, the conventions to follow, and the rules that nobody could infer from the code. The harness adds this file to the context at the start of every session (an instruction file). In a large project, smaller files placed in its folders are added only when the agent works in those folders (scoped instruction files).

What belongs in these files, how much should load at once, what happens when two disagree, and how do you know that a file helps?