docs: setup instructions match what a render contains

[AlexeyShalaev]

The problem

The setup instructions had drifted from what a render contains:

1. The post-generation message ends with "See NEW_LIBRARY_CHECKLIST.md". 7509b31 moved that file out of template/, so no render had it.
2. README.md and .claude/LIBRARY_CREATION.md said to git rm scripts/setup_repo.py .claude/LIBRARY_CREATION.md in the new library. Neither file is in a render, so the command fails.
3. The checklist's manual publish route was to push a tag and create a GitHub Release. Since 4c9f596, publish.yml listens only to Release Please's workflow_run and to workflow_dispatch, so that route ends in a tag and a Release, with nothing published and no error.

The decision

1. The checklist is rendered again (template/NEW_LIBRARY_CHECKLIST.md.jinja). It already renders project-specific commands and tells the reader to delete it, and the "Done" list now ends with that step. I considered keeping it in the template repository and linking to it by URL. A rendered copy has the project's own slug, org and package name in every command, and with Generated projects have no .copier-answers.yml, so copier update cannot bring template fixes into them #14 in place it costs nothing later. I checked with Copier 9.19.1 that a library which deleted the checklist keeps it deleted through copier update, even after the template changes it.
2. The delete instructions are gone. What's left says to run setup_repo.py from the template repository after the first CI pass. That is the checklist's Step 3, and the only way it works. .claude/LIBRARY_CREATION.md §3 now follows that order (generate, commit, create and push, then setup), and its checklists no longer ask to delete files that aren't there.
3. The manual route is the dispatch: gh workflow run publish.yml --ref master -f tag=<slug>-vX.Y.Z. All ten successful manual publishes in the org were done this way. It is also right after fix: publish only reviewed commits that passed CI #18, which keeps the input but only accepts it from master for a tag on master whose CI passed. The checklist now says that pushing a tag or creating a Release publishes nothing.

Evidence

Render of origin/master against a render of this branch:

before: ls: cannot access 'NEW_LIBRARY_CHECKLIST.md': No such file or directory
after:  NEW_LIBRARY_CHECKLIST.md        (no unrendered {{ }} or {% %} left; commands read demo-lib / demo_lib)
both:   scripts/ = emit_markdown.py only, no .claude/   (so the removed git rm could never have worked)

The rendered Step 6 now reads:

gh workflow run publish.yml --ref master -f tag=demo-lib-v0.1.0

The generated gate passes with the checklist present: ruff (which excludes *.md) and mypy are clean, 1 test passed, and uv build builds both files. The checklist is not in the sdist, whose include list is explicit.

Closes #15