ci: run the documentation link checker

Wires scripts/check-doc-links.sh into build-and-test.yml next to the existing
boundary tripwire, and exposes it as `bun run check:links`.

The docs are the maintained source of truth for architecture and process and
cross-reference each other heavily, so a rename that misses a link quietly
turns a doc into a dead end. Pure shell — nothing is installed at job time.

The script itself is landing separately; this job step is red until it does.

.gitea/workflows/build-and-test.yml

@@ -62,6 +62,13 @@ jobs:
       - name: Check frontend/backend boundary
         run: bash scripts/check-frontend-boundary.sh
 
+      # The docs are the maintained source of truth for architecture and
+      # process, and they cross-reference each other heavily. A rename that
+      # misses a link turns a doc into a dead end silently. Pure shell + git —
+      # no tool is installed at job time.
+      - name: Check documentation links
+        run: bash scripts/check-doc-links.sh
+
       - name: Run frontend tests
         run: |
           bunx svelte-kit sync