docs(artifacts): cross-link contribution identifier grammar

Add a direct link from docs/reference/artifacts.md to the contribution-identifiers section of the extension API reference next to the existing presets.md link, so readers of the artifact CLI reference can find the id/lookupId grammar without re-deriving it here.

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Author: nicolehaugen

docs/reference/artifacts.md

@@ -147,7 +147,7 @@ The top-level `id`, `name`, `kind`, `description`, and `stack` fields match the
 
 `active` and `hidden` are independent labels, not opposites. Composing strategies (`wrap`, `prepend`, `append`) keep lower layers in the composed output, so an inactive layer is not necessarily hidden: only layers below the first `replace` layer are marked `hidden`. Built-in rows have no provenance: `layer`, `sourceId`, and `lookupId` are `null` — but `id` is always populated, even on built-in rows. `id` is the round-trip key: `specify artifact info` accepts it as input (for example, `specify artifact info command:speckit.specify --json`), and it resolves the same artifact whether the caller passes the bare name or the `id`.
 
-Lookup IDs use the same grammar as [preset contribution identifiers](presets.md#contribution-identifiers), so a `lookupId` from this command joins directly to `PresetManifest.iter_contributions()` / `ExtensionManifest.iter_contributions()` for manifest-declared layers. Project-local overrides carry a synthetic `project:_:{kind}:{name}` ID that intentionally matches no manifest contribution. `lookupId` is manifest-backed layer provenance, not the round-trip key — use `id` for that. `sourcePath` is populated only when the layer maps to a concrete installed preset/extension file or a tracked agent materialization; core, project-override, and other synthetic rows report `null`.
+Lookup IDs use the same grammar as [preset contribution identifiers](presets.md#contribution-identifiers) and [extension contribution identifiers](../../extensions/EXTENSION-API-REFERENCE.md#contribution-identifiers), so a `lookupId` from this command joins directly to `PresetManifest.iter_contributions()` / `ExtensionManifest.iter_contributions()` for manifest-declared layers. Project-local overrides carry a synthetic `project:_:{kind}:{name}` ID that intentionally matches no manifest contribution. `lookupId` is manifest-backed layer provenance, not the round-trip key — use `id` for that. `sourcePath` is populated only when the layer maps to a concrete installed preset/extension file or a tracked agent materialization; core, project-override, and other synthetic rows report `null`.
 
 ## JSON Errors