chore(schema): add milestone as valid depends_on source type

Milestones can now use depends_on relationships to express that a
milestone depends on concepts, decisions, or other entities. Previously
milestones were restricted to precedes relationships.

Resolves issue #23

Author: Mearman

src/endpoint-types.ts

@@ -61,6 +61,7 @@ export const RELATIONSHIP_ENDPOINT_TYPES: Record<
 			"role",
 			"gate",
 			"mode",
+			"milestone",
 			"artefact",
 			"decision",
 			"change",