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/API.md

@@ -514,7 +514,7 @@ Streams `heartbeat` events every 10 seconds indefinitely. Used as a Pangolin tun
 
 **Event format**
 
-```
+```text
 event: heartbeat
 id: 0
 data: {"ts":"2025-06-10T14:00:00.000Z","id":0}
@@ -541,7 +541,7 @@ A `heartbeat` event is sent every 30 seconds to keep the Pangolin connection ali
 
 **Event format example**
 
-```
+```text
 event: item:added
 id: 1-1718020800000
 data: {"type":"item:added","listId":1,"payload":{"id":11,"listId":1,"text":"Eggs"}}