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)

.markdownlint-cli2.jsonc

@@ -16,8 +16,8 @@
     // Rules DISABLED (Prettier owns these OR they fire on non-author-controlled files):
     "MD041": false, // first-line-h1: CLAUDE.md legitimately starts with ## Project
     "MD034": false, // no-bare-urls: CLAUDE.md version table uses pkg@version syntax
-    "MD036": false // no-emphasis-as-heading: docs/API.md uses **Response 200** as label
+    "MD036": false, // no-emphasis-as-heading: docs/API.md uses **Response 200** as label
   },
   "globs": ["docs/**/*.md", "*.md", "apps/**/*.md"],
-  "ignores": [".planning/**", "node_modules/**", "**/node_modules/**", ".pnpm-store/**"]
+  "ignores": [".planning/**", "node_modules/**", "**/node_modules/**", ".pnpm-store/**"],
 }