ci: trigger CUA smoke test on release tags + document test policy

- Add v* tag trigger to cua-smoke.yml so releases are E2E tested
- Add 'When to run CUA test' section to AGENTS.md documenting mandatory testing

Closes #13 (partial)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/cua-smoke.yml

@@ -4,6 +4,7 @@ on:
   workflow_dispatch:
   push:
     branches: [main]
+    tags: ["v*"]
     paths:
       - "scripts/android-cua-smoke.py"
       - "src/**"