v2.19.0

Activates the skill dependencies declared in 2.18.0. That release shipped the
metadata.boost-requires declarations but they were inert on the engine
available at the time; boost-core 1.4.0 resolves them, so this release raises
the floor to require it.

Changed

• Requires sandermuller/boost-core ^1.4 (raised from ^1.3).
  boost-core 1.4.0 resolves metadata.boost-requires: whenever a skill ships,
  every skill it hands off to ships too, and a required skill that a consumer's
  tags would otherwise drop is rescued in (transitively, surfaced as an INFO
  diagnostic). Pinning the floor here makes the co-shipping guarantee real for
  every consumer instead of best-effort. 1.4.0 is additive and backward
  compatible, and the catalog already required ^1.3, so the step is small.
  Authoring guidance for boost-requires lives in boost-core's README.

No skill content changed — the declarations themselves shipped in 2.18.0.

Full Changelog: 2.18.0...2.19.0