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/ARCHITECTURE.md

@@ -61,7 +61,7 @@ graph TD
 
 ## Directory Structure
 
-```
+```text
 familysync/
 ├── apps/
 │   ├── api/                  # Hono backend (Node 22 + TypeScript)
@@ -168,7 +168,7 @@ familysync/
 
 ### Backend modules
 
-```
+```text
 routes/events.ts  ──→  broker/expand.ts     (read: RRULE expansion)
                   ──→  calendarOutbox (DB)   (write: enqueue)
                   ──→  broker/sync.ts        (write-sync after outbox drain)