feat: provider resource declarations, kind-aware routing, MCP dynamic schema

.changeset/provider-resource-declaration.md

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+---
+'@refkit/core': minor
+'@refkit/mcp': minor
+'@refkit/provider-testkit': minor
+'@refkit/provider-unsplash': minor
+'@refkit/provider-pexels': minor
+'@refkit/provider-pixabay': minor
+'@refkit/provider-flickr': minor
+'@refkit/provider-brave': minor
+'@refkit/provider-wikimedia-commons': minor
+'@refkit/provider-openverse': minor
+'@refkit/provider-met': minor
+'@refkit/provider-artic': minor
+'@refkit/provider-rijksmuseum': minor
+'@refkit/provider-smithsonian': minor
+'@refkit/provider-europeana': minor
+'@refkit/provider-freesound': minor
+'@refkit/provider-jamendo': minor
+'@refkit/provider-gutendex': minor
+'@refkit/provider-poetrydb': minor
+'@refkit/provider-internet-archive': minor
+'@refkit/provider-polyhaven': minor
+'@refkit/provider-nailbook': minor
+---
+
+Provider resource declarations: open `ResourceKind` vocabulary with optional
+`kinds` + `description` on providers and `kind` on references; declaration-gated
+kind routing (a kind-filtered search skips providers whose declared `kinds`
+lack the value, with a new `unsupported-kind` skip reason) composed with the
+existing `sources` id filter; MCP tool schema, per-provider source list, and
+the `modalities` / `media.kind` enums now derived from registered provider
+declarations at startup.
+
+Note: the MCP `modalities` input enum is now deployment-dependent (derived
+from the registered providers). A request naming a modality no registered
+provider supports is now rejected at the schema boundary — previously a
+fully-unsupported request threw at search time, and a mixed request (e.g.
+image+audio against an image-only deployment) returned the supported subset.
+Fail-loud at the boundary is intentional.