feat: export versioned schemas in package.json

Add exports for versioned schemas to allow consumers to import:
- ./schemas: All versioned schemas (v0_1, v0_2, v0_3, latest)
- ./schemas/*: Specific schema versions (./schemas/0.1, ./schemas/any, etc.)
- ./schemas-loose: Loose validation schemas

This enables cleaner imports like:
import { McpbManifestSchema } from '@anthropic-ai/mcpb/schemas/any'
import * as v0_2 from '@anthropic-ai/mcpb/schemas/0.2'
import * as schemas from '@anthropic-ai/mcpb/schemas'

package.json

@@ -27,7 +27,22 @@
       "import": "./dist/cli.js",
       "require": "./dist/cli.js"
     },
-    "./mcpb-manifest.schema.json": "./dist/mcpb-manifest.schema.json"
+    "./mcpb-manifest.schema.json": "./dist/mcpb-manifest.schema.json",
+    "./schemas": {
+      "types": "./dist/schemas/index.d.ts",
+      "import": "./dist/schemas/index.js",
+      "require": "./dist/schemas/index.js"
+    },
+    "./schemas/*": {
+      "types": "./dist/schemas/*.d.ts",
+      "import": "./dist/schemas/*.js",
+      "require": "./dist/schemas/*.js"
+    },
+    "./schemas-loose": {
+      "types": "./dist/schemas-loose.d.ts",
+      "import": "./dist/schemas-loose.js",
+      "require": "./dist/schemas-loose.js"
+    }
   },
   "bin": "dist/cli/cli.js",
   "files": [