Add a slash comment for fixing up precommit

Author: nedtwigg

.claude/commands/precommit.md

@@ -0,0 +1,4 @@
+1. run `pnpm precommit`
+2. if tests fail, run `pnpm test -u` might fix them
+3. fix other problems manually
+4. go back to step 1: `pnpm precommit`