> I’ve been having decent luck telling it to keep track of itself in a .plan file, not foolproof, of course, but it has some ability to “preserve context” between contexts.

This is the way. Not only have I had good luck with both a TASKS.md and TASKS-COMPLETE.md (for history), but I have an .llm/arch full of AI-assisted, for-LLM .md files (auth.md, data-access.md, etc.) that document architecture decisions made along the way. They're invaluable for effectively and efficiently crossing context chasms.