refactor(skills): share style skill instructions

src/skills/style-skill.md

@@ -0,0 +1,29 @@
+# Style skill instructions
+
+Use this document for skills that own repository style conventions.
+
+## Load rules
+
+Read every Markdown file directly under `rules/` before applying the skill.
+Resolve `rules/` relative to the skill's `SKILL.md`.
+
+Finish when all discovered rule files have been read.
+
+## Apply rules
+
+Apply the loaded rules when editing or reviewing files covered by the skill.
+When loaded rules conflict, follow the most specific rule.
+
+After editing files covered by the skill, run:
+
+```bash
+nix run .#fix
+```
+
+Finish when `nix run .#fix` succeeds and the change follows the loaded rules.
+If the command fails, report the failure.
+
+## Maintain rules
+
+When adding or changing files under `rules/`, read the shared rule format at `src/skills/RULE-FORMAT.md` first.
+Finish when the changed rule files follow that format.