Add ErrorContext interface to UnitResult so error information (provider
errors, timeouts, idle watchdog kills) is no longer discarded at the
resolve boundary. The four call sites that previously threw away context
now attach typed error metadata with category, message, and transience.
Downstream consumers (stuck detection in phases.ts, journal unit-end
events) use the structured errorContext field directly instead of
fragile regex heuristics on message content.
dc723b2519