Adds the single exception type used throughout the pipeline. Every failure
maps to one of the ten IX_* codes from the MVP spec §8 with a stable
machine-readable code and an optional free-form detail. The `str()` form is
log-scrapable with a single regex (`IX_xxx_xxx: <msg> (detail=...)`), so
mammon-side reliability UX can classify failures without brittle string
parsing.
Enum values equal names so callers can serialise either.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
