refactor: Update agent-sdk-dev plugin structure and configuration (#9230)

- Move plugin.json to .claude-plugin directory to match pr-review-toolkit pattern
- Add author information: Ashwin Bhat (ashwin@anthropic.com)
- Update model field from sonnet-4.5 to sonnet in all agent/command definitions

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

Co-authored-by: Claude <noreply@anthropic.com>

plugins/agent-sdk-dev/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "agent-sdk-dev",
+  "description": "Claude Agent SDK Development Plugin",
+  "version": "1.0.0",
+  "author": {
+    "name": "Ashwin Bhat",
+    "email": "ashwin@anthropic.com"
+  }
+}