You're right — the current approach has a fundamental evolution problem.
The issue isn't just where feedback is stored. It's that a monolithic prompt is a black box — when it picks the wrong post, you don't know which step of the reasoning failed. Was it bad at judging niche fit? Bad at spotting engagement opportunity? Bad at matching your expertise? You can't tell, so you can't fix it precisely. Feedback goes into a general "try harder next time" pile that doesn't map to anything structural.
Why LangGraph is the right direction
LangGraph forces you to break the agent into explicit nodes with observable, structured outputs. Each node reads from shared state and writes back to it. This means every intermediate decision is logged — not just the final post pick.