fix(13-03): exclude .planning from Prettier so the format gate stays green

GSD planning artifacts (.planning/**) are rewritten by tooling on every workflow
run and are never hand-formatted. Including them in Prettier scope caused
pnpm format:check to exit 1 on virtually every future PR, making the SC-3
green baseline impossible to hold. Add .planning/ to .prettierignore, mirroring
the same rationale as dist/ and apps/api/src/db/migrations/.

Real source docs (README.md, docs/**) remain in Prettier scope.
No playwright-report/, test-results/, or coverage/ directories exist to ignore.

.prettierignore

@@ -3,3 +3,6 @@ node_modules/
 pnpm-lock.yaml
 apps/api/src/db/migrations/
 *.html
+# GSD planning artifacts — tool-generated and rewritten on every workflow run;
+# never hand-formatted; must not gate Prettier (same rationale as dist/).
+.planning/