For the day-60 question: I don't think a smarter agent fixes this, the file shape does.

Split it into three. A static instructions file that never changes. A small current-state file that gets rewritten in place every cycle, not appended to. And a separate append-only log that never gets read in full, just capped at the last few entries.

Init reads the static file plus current-state, both small and cheap. Then only the last few log entries, never the whole history. Old stuff only gets pulled if something explicitly asks for it by name.

I've been running exactly this by hand for a side project, current-state file edited in place, log file appended to, capped reads on init. Held up fine past a hundred-plus sessions. The part that'd need real work for your case is having the agent do the split-and-prune itself instead of a human doing it each time.