docs: warn against background until-loops for non-existent .completed markers

.claude/skills/running-tend/SKILL.md

@@ -27,6 +27,26 @@ permission first) still applies when the target shows no agent signals.
   in #5753, so bot PRs must be merged manually by a maintainer (or via repo
   branch-protection auto-merge if a maintainer enables it on the PR).
 
+## Background tasks
+
+When a Bash command exceeds its tool timeout, the harness auto-backgrounds it
+and the tool result reads
+`Command running in background with ID: bgXXXX. Output is being written to: …/bgXXXX.output. You will be notified when it completes.`
+The notification arrives as a `<task-notification>` user message — there is **no
+`.completed` marker file**, only the `.output` file.
+
+Do not start a `run_in_background: true`
+`until [ -f …/bgXXXX.completed ]; do sleep N; done` loop to wait for it. That
+loop polls forever (the marker never appears), and because it's running in
+background the session can't reap it. When the session ends, the orphan shell +
+sleep keep the runner alive until the workflow's `timeout-minutes` cap, which
+converts an otherwise successful run into `cancelled` (see
+[max-sixty/tend#594](https://github.com/max-sixty/tend/issues/594) for a 5h40m
+incident on this repo).
+
+Instead: continue with other work after the auto-background. The task
+notification re-enters the session; act on it then.
+
 ## Weekly maintenance
 
 These tasks run as Step 3 of the bundled weekly skill (only when