Calendar commands now return structured, typed error envelopes for every
failure mode — input validation, internal faults, and API responses —
instead of legacy generic errors. Callers and AI agents get consistent
exit codes and a machine-readable shape (type / subtype / code / hint),
and can tell bad input, an internal fault, and an API rejection apart.
Validation errors are attributed to the offending flag.
Server-supplied error details (e.g. why an event time was rejected) are
surfaced on the typed error's hint via a shared classifier improvement
that benefits every domain. Multi-step operations (create-with-attendees
rollback, multi-field update) preserve the real failure's classification
and report which steps completed.
The whole calendar domain is now lint-locked against reintroducing legacy
error constructors.