Fix Step 1 update-mode scratch file and stale example-skills-catalog reference

[dmccoystephenson]

Summary

• Step 1's update-mode diff referenced /tmp/<slug>-dev-loop.new.md without ever saying how that file is produced. The natural reading was shell > redirection into /tmp -- exactly the pattern PR Add Tests-fix CI-revert fallback ladder and scratch-file guidance #67 documented as blocked in some harness sandboxes, and /tmp is outside the repo directory some sandboxes restrict even when in-repo writes succeed. If the scratch write silently failed, the pre-overwrite diff -- the only confirmation gate protecting an existing skill file -- could be skipped.
• Now: names the Write tool as the mechanism, uses an in-repo scratch path (~/local-skills/<slug>-dev-loop/<slug>-dev-loop.md.new), and fails closed -- abort and report rather than overwrite without having shown the diff -- if the scratch write fails. Cleans up with python3 -c "import os; os.remove(path)" rather than rm, matching PR Add Tests-fix CI-revert fallback ladder and scratch-file guidance #67's rationale.
• Fixed a second, smaller drift found while in this section: the MODE=update bullet still said "record in example-skills-catalog" after PR Make example-skills-catalog catalog recording optional #69 renamed Step 7 to an optional personal catalog and made it conditional on the directory existing.

Closes #73

Note on scope

create-dev-loop.md is on the do-not-auto-merge list -- issue #73 explicitly flagged that this needs human review before merge and validation via a MODE=update end-to-end run per CLAUDE.md's "Testing changes" item 5. Opening under the maintainer's direct, in-session request to polish the repo.

Research grounding

No RESEARCH.md finding applies -- this is the same class of harness/sandbox operational observation PR #67 explicitly declined to force a citation for (matches issue #73's own assessment).

Test plan

• python3 scripts/check_docs.py passes
• No {{placeholder}} added or changed; no Step count/numbering change
• UNVERIFIED -- a full MODE=update end-to-end dry run against a fixture repo with an existing generated skill was not performed. This session has no fixture repo with a pre-existing <slug>-dev-loop skill to update against, and the sandbox restricts filesystem writes outside this repo's own working directory (the same constraint this PR documents). The change is prose-only inside an existing Step (no placeholders, no fence/escaping changes, no Step renumbering), which lowers risk, but a maintainer with a real fixture should confirm MODE=update still produces a reviewable diff and correctly cleans up the scratch file before treating this as fully verified.

---

drafted by Claude on behalf of Daniel Stephenson