add agent rules

.opencode/rules/formatting.md

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+# Formatting
+
+## Command
+
+- `pnpm format` writes formatted code (Prettier).
+- `pnpm lint` verifies formatting plus ESLint; use it as the check.
+
+## Prettier
+
+- `.prettierrc` is the source of truth for all Prettier settings. Read it rather than assuming defaults; if its values change, follow the file.
+- Do not hand-format to match remembered settings — run `pnpm format` and let Prettier decide.
+- `.prettierignore` lists files Prettier skips (lockfiles, `build/`, `node_modules/`, etc.). Respect it.
+
+## Tailwind class ordering
+
+- `prettier-plugin-tailwindcss` sorts utility class names automatically. Never manually reorder classes for sorting — just run `pnpm format`.