Replaces the pkg/errors fork with a thin wrapper over the standard
library's errors package. A single withStack struct captures stack
traces via runtime.Callers, while fmt.Errorf("%w", ...) handles all
message wrapping. This enables errors.Is/As/Unwrap chain traversal
(the fork silently broke them) and deletes ~190 lines of stack/frame
formatting code. Adds table-driven tests at 100% statement coverage.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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