I like a lightweight ADR system.

e.g. docs/decisions/README.md (index with a blurb about each decision), docs/decisions/01-some-lesson.md (some architectural decision/pattern that you or the agents discovered).

ADR files have important sections like "rejected solutions" and "acceptable risks", and they're live files that can be refined and pivoted over time or retired to docs/decisions/archive/.

It's also nice to give each top-level bullet point some stable ID like "R1" for rejected solution #1, I3 for invariant #3. Agents use this stuff intelligently all the time like "This could be a time to reconsider D4/R2" = ADR #4, rejected solution #2.

The essential part being that your system ratchets into increasingly better decisions and invariants over time, and there's a place to actually put this stuff.

It's essential for automating high-quality software and something we couldn't be arsed to do much less update before AI.