planning: add issue 2.1-add-tee-slow-output-convention

Author: cowwoc

.cat/issues/v2/v2.1/add-tee-slow-output-convention/index.json

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+{
+  "status": "open"
+}

.cat/issues/v2/v2.1/add-tee-slow-output-convention/plan.md

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+# Plan
+
+## Goal
+
+Add a plugin convention requiring that slow processes (builds, test suites, linters) have their output tee'd to a
+temporary file so the output can be re-filtered without re-running the command.
+
+## Pre-conditions
+
+(none)
+
+## Post-conditions
+
+- [ ] A new file `plugin/rules/tee-slow-output.md` exists with the convention
+- [ ] The rule includes correct frontmatter (`mainAgent: true`, `subAgents: [all]`)
+- [ ] The rule explains when to tee (slow commands), when not to (fast commands, background tasks), and cleanup expectations
+- [ ] The rule includes a concrete pattern example using `mktemp` and `tee`