v4-v6 upgrade improvement and warning about file auto backup

bmad/bmm/testarch/knowledge/error-handling.md

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+# Error Handling and Resilience Checks
+
+- Treat expected failures explicitly: intercept network errors and assert UI fallbacks (`error-message` visible, retries triggered).
+- In Cypress, use scoped `Cypress.on('uncaught:exception')` to ignore known errors; rethrow anything else so regressions fail.
+- In Playwright, hook `page.on('pageerror')` and only swallow the specific, documented error messages.
+- Test retry/backoff logic by forcing sequential failures (e.g., 500, timeout, success) and asserting telemetry gets recorded.
+- Log captured errors with context (request payload, user/session) but redact secrets to keep artifacts safe for sharing.
+
+_Source: Murat error-handling patterns, Pact resilience guidance._