feat(15-01): fix 13 markdown violations + wire md:lint to fast-checks CI step

- Fix MD040 (11 bare fences): add language tags (text/bash) across 7 files
- Fix MD031 (2 violations): add blank lines around fence in GETTING-STARTED.md
- Wire 'Markdown lint' step to fast-checks job (after Format check, before Typecheck)
- Reformat .markdownlint-cli2.jsonc per Prettier (trailing commas in JSONC)
- pnpm md:lint exits 0; pnpm format:check exits 0; gate can fail on bare fence (verified)

docs/DEVELOPMENT.md

@@ -6,7 +6,7 @@ Local development setup and workflows for FamilySync — a pnpm monorepo with tw
 
 ## Repo Layout
 
-```
+```text
 familysync/
 ├── apps/
 │   ├── api/          # Hono API server — Node.js 22, TypeScript, Drizzle/MariaDB
@@ -19,7 +19,7 @@ familysync/
 
 Key paths inside `apps/api/src/`:
 
-```
+```text
 src/
 ├── db/
 │   ├── schema.ts       # Drizzle table definitions (source of truth for migrations)