- Extend empty-description validation to error codes

- Move to endpoint-libs 2.0 (lockstep)
- Depend on published endpoint-libs 2.0.0-alpha.1

**Behaviour change:** generation now fails on missing or whitespace-only
descriptions for endpoints, enum variants and error codes. These become MCP
tool metadata, generated doc comments and the error-code reference, so a blank
one produces a tool an agent cannot use correctly.

Pass `--allow-empty-descriptions` to restore the previous behaviour.

Requires `endpoint-libs 2.0.0-alpha.1`. That is an exact pre-release pin — a
plain `"2.0"` requirement would not match it — so upgrading to a final 2.0.0
will need a deliberate bump here.