fix(reusable-lint): pin prettier to the repo-root config

[bryanfawcett]

A nested .prettierrc that names a plugin kills the whole prettier job.

The bug, found in production

nyuchi-identity's apps/auth-ui is a SvelteKit app with its own auto-discovered .prettierrc declaring prettier-plugin-svelte and prettier-plugin-tailwindcss. The gate installs prettier globally and never runs npm ci, so walking into that directory kills the entire job, not just that subtree:

[error] Cannot find package 'prettier-plugin-svelte' imported from <repo>/noop.js

Any monorepo with a per-app prettier config naming a plugin does the same.

Why no in-repo fix was acceptable

option	why not
delete apps/auth-ui/.prettierrc	breaks the app's own formatting — without the svelte plugin prettier cannot parse a .svelte file at all
add apps/auth-ui/ to .prettierignore	narrows the gate, which is the thing we are eliminating
leave both	live trap — the app's own prettier --write . re-tabs package.json and turns the org check red next time anyone formats

The fix

Pass --config .prettierrc, exactly symmetric with the --ignore-path argument already there and for the same reason: this job must use the org's settings, not whatever it auto-discovers walking the tree. A sub-app keeps its own .prettierrc for its own scripts and editors; it just cannot redefine the org gate from a subdirectory.

A missing root .prettierrc is now a loud error naming the fix, rather than prettier silently falling back to defaults.

Proven both ways

case	before	after
nested config naming a missing plugin	[error] Cannot find package 'prettier-plugin-svelte', whole job dies	clean
badly formatted apps/auth-ui/README.md	—	still caught — gate is not narrowed
missing root .prettierrc	silent fallback to defaults	loud error, exit 1

Unblocks nyuchi/nyuchi-identity#14.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com