When main session + sub-agents both edit a shared task file, the edit
tool's exact-match requirement causes silent failures. Solution: split
into an immutable goals file (main session only) and an append-only
task log (sub-agents only append, never edit).
Also adds tip to keep AUTONOMOUS.md token-light by archiving completed
tasks to a separate file loaded only on-demand.
Verified in production over ~1 day of autonomous heartbeat workflows.
