Implement VS Code native MCP event system for automatic tool refreshing

- Added HumanAgentMcpProvider class with onDidChangeMcpServerDefinitions event
- Registered MCP provider with VS Code lm.registerMcpServerDefinitionProvider
- Added mcpServerDefinitionProviders contribution point to package.json
- Implemented startup event firing when override files exist
- Fixed session name loading timing by moving after server startup with retry
- Chat webview now fires MCP events when override files are reloaded
- VS Code Configure Tools will now automatically refresh tool descriptions

package.json

@@ -108,6 +108,12 @@
         }
       ]
     },
+    "mcpServerDefinitionProviders": [
+      {
+        "id": "humanagent-mcp.server",
+        "label": "HumanAgent MCP Server"
+      }
+    ],
     "configuration": {
       "title": "HumanAgent MCP",
       "properties": {