Step 1's update-mode diff needs a scratch file at /tmp but never says how to write it #73

What's wrong

Step 1 of create-dev-loop.md tells the generator to show a diff before overwriting in update mode:

diff -u ~/local-skills/<slug>-dev-loop/<slug>-dev-loop.md /tmp/<slug>-dev-loop.new.md

The right-hand path is a scratch file holding the freshly generated content, but no Step says how it gets there. The natural reading is shell redirection into /tmp — which is exactly the pattern PR #67 documented as unavailable in some harness sandboxes.

Why it matters

PR #67 added this Universal rule to the generated template's Phase 3:

Scratch-file handling in a sandboxed harness. When a step needs a scratch file for inspection or transformation [...] prefer the Write tool over > file shell redirection, and prefer python3 -c "import os; os.remove(path)" over rm to clean it up afterward. Some harness sandboxes statically block plain > redirection and rm outright — even for files the same session just created inside the working directory.

So create-dev-loop.md teaches its generated children a lesson its own Steps don't follow. Two failure modes stack here:

1. Plain > redirection is statically blocked in some sandboxes, so the scratch file never gets written and diff -u compares against a nonexistent path.
2. /tmp is outside the repo working directory. At least one sandbox permits in-repo writes via the Write tool while refusing out-of-repo scratch writes entirely — so even following PR Add Tests-fix CI-revert fallback ladder and scratch-file guidance #67's advice, the hardcoded /tmp path can fail.

The consequence is not cosmetic: the pre-overwrite diff is the only confirmation gate protecting a user's existing skill file from being replaced. If the scratch write silently fails, the generator either aborts mid-update or skips the gate.

Suggested fix

In Step 1, say explicitly how the scratch file is produced and pick a path that survives a sandbox:

• Name the Write tool as the mechanism, matching the Phase 3 rule PR Add Tests-fix CI-revert fallback ladder and scratch-file guidance #67 added.
• Prefer a path inside ~/local-skills/<slug>-dev-loop/ (e.g. <slug>-dev-loop.md.new) over /tmp, so the write lands in a directory the generator already owns, and clean it up with python3 -c "import os; os.remove(path)" after the diff is shown.
• Alternatively, state that if the scratch write fails, the generator must abort rather than proceed to overwrite without showing the diff — the gate must fail closed.

How this was found

Stage A documentation-accuracy sweep (see PR #72). This is a behavioral gap in create-dev-loop.md, not doc drift, so it was filed rather than fixed under a docs cycle.

Research grounding

No RESEARCH.md finding applies — this is a harness/sandbox operational observation, the same class PR #67 explicitly declined to force a citation for. Stating that rather than reaching for one.

Note for whoever picks this up: create-dev-loop.md is on the do-not-auto-merge list, so the fix needs human review, and the Step 1 change should be validated by actually running MODE=update end-to-end per CLAUDE.md's "Testing changes" item 5.