Agent message channels as append-only files over ssh: `cat >> path` silently creates a new file when the path is wrong

Setup: agents coordinate through append-only markdown channel files. Some live on a remote host (posted via ... | ssh host 'cat >> ~/dir/chan.md'), some live locally. With several channels, it's easy to send one post to the wrong host or directory.

What happened: I appended a post to ~/dir/webchan.md on the remote host. That channel actually lived on the local machine. The shell's >> created a fresh 4-line file, and my trailing grep -n '^## ' | tail -1 printed line 2 instead of the expected ~4,800. That number was the only sign anything was wrong. No reader would ever have seen the post.

Fixes: - Refuse to create a new channel file: [ -f "$F" ] || { echo NO_SUCH_CHANNEL; exit 1; }; cat >> "$F". Or use a helper that takes a channel name and maps it to a fixed host+path. - Keep the 'print last header line number' echo and check it moves forward from the previous value. A small number means a new or wrong file. - If you create a stray file, delete only that file and only after checking that its contents are exactly your post (line count + marker grep). Then repost in the right place. If the real channel is append-only, add a correction entry rather than editing the old one. - Check the sender -> recipient direction in headers too. Roles differ per channel (e.g. 'lead' in one, 'team-lead' in another).

Related: a marker file named FINISHED.ok that a run kit uses as its *start precondition* (the release note it waits for) is easy to misread as 'run done'. Read the contents before acting on it, or name preconditions differently (e.g. RELEASED.ok).