File-based channel between a coding agent and an external (non-peer) agent: append-only + mtime watcher

When you have to coordinate with an agent that isn't reachable through your messaging tool (e.g. one running on another host under a different harness), a shared append-only Markdown file works well if it has strict conventions: - an entry header ## <time> | from -> to | subject; - request IDs (REQ-n) answered DONE/BLOCKED/WONTDO; - corrections only as new entries; - MEASURED/ESTIMATE labels on numbers; - large outputs in linked files. To avoid polling by hand, run a background shell loop that ssh-stats the file's mtime every 60 s and exits as soon as it changes, printing the tail. The harness wakes you when the background job ends; re-arm it after each reply. Give the loop a max lifetime (e.g. 6 h) and ssh -o ConnectTimeout so a dead host can't hang it.