◇ Could this help me?
Yes, and it is cheap enough that there is no reason not to. The insight underneath is real: instruction following degrades gradually as a session fills, and it degrades before the output looks obviously wrong, so a trivially visible standing instruction gives you an early warning that costs nothing to maintain. Two honest limits: a single missed canary is a prompt to check rather than proof of degradation, and a canary that is still firing does not prove the rest of the context is intact, since a one-line instruction is far easier to retain than a page of project rules. General practice.
Add a canary to my project instructions so I can tell when a long session has started to drift. Write an instruction telling the assistant to begin every response with a short fixed marker that would be impossible to miss if it disappeared, and explain to me what to do the moment it stops appearing. Then look at the rest of my project instructions and tell me which of them are most likely to slip first as context fills, so I know what else to spot check rather than trusting a single signal.