Make McpbManifest point to v0.2 for backwards compatibility

- Change McpbManifest type to point to v0.2 schema instead of McpbManifestAny
- Add DEFAULT_MANIFEST_VERSION export to browser.ts
- Update test fixtures to use manifest_version "0.2" when using McpbManifest type
- Keep @deprecated annotation directing users to McpbManifestAny for multi-version support

This maintains backwards compatibility for existing code using McpbManifest while encouraging new code to use McpbManifestAny for broader version support.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/config.test.ts

@@ -90,7 +90,7 @@ describe("getMcpConfigForManifest", () => {
   };
 
   const baseManifest: McpbManifest = {
-    manifest_version: "0.3",
+    manifest_version: "0.2",
     name: "test-extension",
     version: "1.0.0",
     description: "Test extension",
@@ -305,7 +305,7 @@ describe("getMcpConfigForManifest", () => {
 
 describe("hasRequiredConfigMissing", () => {
   const baseManifest: McpbManifest = {
-    manifest_version: "0.3",
+    manifest_version: "0.2",
     name: "test-extension",
     version: "1.0.0",
     description: "Test extension",