chore: enforce linting as part of build

AGENTS.md

@@ -30,6 +30,11 @@
    - Iterate toward partial progress while keeping the failure scoped and observable (Orange).
    - Finish only when the behavior is implemented and the test loop passes cleanly (Green).
 
+7. **Linting is part of the build, not an optional extra step.**
+   - `make build` must enforce linting first.
+   - Do not treat lint as a separate, skippable preflight.
+   - If the build is green, lint must already be green.
+
 ## Project Learnings
 
 ### NiceGUI layout constraint