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)

README.md

@@ -75,7 +75,7 @@ The API listens on port 3000. The PWA build is served separately (Vite `preview`
 
 ## Monorepo Structure
 
-```
+```text
 apps/
   api/        Hono backend — CalDAV sync, OIDC auth, lists API, push notifications
   pwa/        React 19 PWA — calendar view, lists UI, service worker
@@ -116,7 +116,7 @@ docker-compose.dev.yml   Dev overrides (bind-mount src/, expose DB/Redis ports)
 
 FamilySync reads and writes calendars via CalDAV against Fastmail — not JMAP (not available for Fastmail calendars). Configure your Fastmail app password under the "Mail, Contacts & Calendars" scope. The principal URL follows the pattern:
 
-```
+```text
 https://caldav.fastmail.com/dav/principals/user/<your-fastmail-address>/
 ```