The production code changes are all correct — errors.Is is the idiomatic pattern for wrapped error checking since Go 1.13.

However, I found two reversed argument bugs in the test files:

1. pkg/providers/error_classifier_test.go: errors.Is(inner, fe.Unwrap()) should be errors.Is(fe.Unwrap(), inner). The first argument is the error chain to search, the second is the target.

2. pkg/tools/message_test.go: errors.Is(sendErr, result.Err) should be errors.Is(result.Err, sendErr). We're asking "does result.Err match sendErr?", not the reverse.

Both happen to work by coincidence (since the objects are equal), but the semantics are wrong and could mask real bugs if the code is later modified to wrap errors.

Also: the copy -> cp rename in codex_provider.go is unrelated — consider a separate commit or mention in description.

Correct and idiomatic. All replacements are appropriate:

• context.Canceled and context.DeadlineExceeded can be wrapped by libraries, so errors.Is is the right approach.
• http.ErrServerClosed — same reasoning.
• syscall.EBUSY — syscall.Errno implements error, so errors.Is works here too, though Errno values are typically not wrapped. Still correct and future-proof.

The test updates (fallback_test.go, message_test.go, error_classifier_test.go) are consistent with the production code changes.

LGTM.