ci: run editorconfig-checker

[dangreen]

Why

The editorconfig job installs editorconfig-checker but never runs it: the main branch of editorconfig-checker/action-editorconfig-checker only adds the binary to PATH, and the example workflow of the action runs it in a step of its own. The job moved from @v1, a Docker action that ran the check by itself, to @main in eceaf52 without such a step, so it has not checked a file since June 2023.

Run as is, editorconfig-checker v4.0.2 reports 3 errors, all from its indent-size rule, which wants every left-padding to be a multiple of indent_size. They are lines of numbered list items in skills/codex/SKILL.md and skills/project/SKILL.md, which markdown aligns with the text of the item, 3 spaces after 1. , and a wrapped row of the file table in skills/project/SKILL.md.

What

• Numbered list items with nested content are written as 1. followed by two spaces, so the content sits at 4 spaces instead of 3. The second column of the file table in skills/project/SKILL.md moves one space right, to an even column.
• The step with the action is now named Install editorconfig-checker, and a new Check editorconfig step runs editorconfig-checker.

Notes

• .editorconfig stays as it is, so every file is still checked against indent_size.
• skills/codex/SKILL.md parses to the same tree as before, and skills/project/SKILL.md differs in the file table alone, checked with mdast-util-from-markdown and its GFM extension.
• With these changes, editorconfig-checker v4.0.2 finds no errors on main.