This kind of specification applies to the agent behavior or the agent behavior in a project. You don't write it on every conversation, you write/review it once for the project or agent and let the harness include it on every conversation. You can even ask the agent to scan the codebase and pull desirable patterns out of it for future sessions.

The entire point of using an agent is to not have to write everything yourself, so that when you write "implement feature X", the feature gets implemented in a way that makes sense in that project, is reviewed, refactored, and tested by agents, and it's good enough to bring in a human reviewer.