Is bin/create-pr still wanted? Unreferenced, unlinted, Bash 4 style

[Chemaclass]

bin/create-pr is a 476-line vendored copy of Chemaclass/create-pr v0.10.

• No references anywhere: not in the Makefile, .github/workflows/, the pr skill, package.json, or the docs. (docs/examples.md links the upstream project, not this file.)
• Having no .sh extension, it was never linted by make sa until docs: stop instructing shfmt -w, and scope make sa correctly #863; it is written in Bash 4 style ([[ ]] throughout), which the project's own compatibility rules prohibit for src/.

Plausibly a deliberate maintainer convenience, which is why the #862 audit left it in place. Worth an explicit decision: keep and document it, or drop it in favour of the upstream tool.