chore(cursor): add ASCII safety rule and refine shell guidance (#5268)

Add text-ascii-safety forbidding AI typography and hidden Unicode, with
portable review commands (shell loops instead of GNU xargs -r) and rg
exclusions for 3rdparty, dist, and .git.

Refine shell-style-guide YAML shell inference when shell: is omitted
(Alpine sh, Ubuntu bash, default sh). Make shell-style-guide prose
ASCII-only and add negated globs on file-scoped rules; mirror vendor and
build path exclusions on testing-guide.

Allow the tracked .mdc rules through .gitignore exceptions.

Author: geyslan