docs: scratch files go in .sx-tmp/, not /tmp (avoids approval prompts)

CLAUDE.md

@@ -339,6 +339,10 @@ files; any can be advanced independently.
 
 ## While working
 
+- **Scratch files go in `.sx-tmp/` (in the repo root), never `/tmp`.** `/tmp`
+  is outside the workspace, so every write there triggers an approval prompt;
+  `.sx-tmp/` is gitignored and approval-free. `mkdir -p .sx-tmp` once, then write
+  probes/snippets there (e.g. `.sx-tmp/probe.sx`).
 - Work on **one step at a time**. Complete it fully before moving on.
 - After completing a step, immediately update the relevant checkpoint
   (`current/CHECKPOINT.md` for IR, `current/CHECKPOINT-FFI.md` for FFI):