Strengthen Copilot review instructions to use imperative "Flag any" phrasing (#62584)

Copilot's review agent converts rules phrased as "Flag any X" into
active code searches but treats passive phrasing as informational
context. Rewrite all critical rules (imports, assert, time.time,
lru_cache, session.commit, N+1 queries, unittest, unspec'd mocks)
to use imperative directives so the agent reliably flags violations.