feature: add tee-slow-output convention to plugin rules

Author: cowwoc

plugin/rules/tee-slow-output.md

@@ -0,0 +1,39 @@
+---
+mainAgent: true
+subAgents: [all]
+---
+## Tee Slow Process Output
+
+**MANDATORY:** When running a slow command (build, test suite, long-running script), capture its output with `tee` so
+you can re-filter the output later without re-running the command.
+
+**Rationale:** Slow commands can take minutes to complete. If you need to search the output for a different pattern or
+review a different section, re-running the command wastes significant time. Capturing the output to a log file lets you
+`grep`, `head`, `tail`, or otherwise re-filter the results instantly.
+
+**Pattern:**
+
+```bash
+# 1. Create a temporary log file
+LOG_FILE=$(mktemp /tmp/build-output-XXXXXX.log)
+
+# 2. Run the slow command with tee to capture output
+mvn -f client/pom.xml test 2>&1 | tee "$LOG_FILE"
+
+# 3. Later, re-filter without re-running the command
+grep -i "error" "$LOG_FILE"
+grep "FAILED" "$LOG_FILE"
+tail -50 "$LOG_FILE"
+```
+
+**When NOT to tee:**
+- **Fast commands** (< 2 seconds) — the overhead of managing a log file outweighs the benefit
+- **Trivially small output** — if the full output fits in context, there is nothing to re-filter
+- **Commands in `run_in_background`** — background task output is already captured and retrievable
+
+**Cleanup:** Delete the log file when you no longer need it. Do not leave temporary log files behind after the task is
+complete.
+
+```bash
+rm -f "$LOG_FILE"
+```