[plugins] Phase 5a: registry/types.Plugin (THV-0077)

[JAORMX]

Goal: Add the registry Plugin type to toolhive-core so both toolhive and toolhive-registry-server can surface plugins in the catalog.

Deliverables:

• registry/types/plugins_types.go — Plugin type + schema-validation hooks (template skills_types.go).

Dependency/gate: independent of P1–P4. Precedes GATE-C3 (tag toolhive-core, e.g. v0.0.27; bump go.mod in both toolhive and toolhive-registry-server to the same tag). This is the cross-repo fan-out gate.

Testing/exit gate: marshal/round-trip + schema-validation tests.

Part of stacklok/toolhive#5525
Refs RFC PR stacklok/toolhive-rfcs#77