CopilotAH: support more custom agent properties

When we read the custom agent files, we read properties from the header: 'model', 'skills', 'infer', 'tools' and pass agents as `customAgents?: CustomAgentConfig[];`
https://github.com/microsoft/vscode/blob/af7386688ddfec0bc8ad1d1de25475309fd8b07d/src/vs/platform/agentHost/node/copilot/copilotPluginConverters.ts#L127

It looks that internally Copilot supports more properties, e.g. 'reasoning-effort'
https://github.com/github/copilot-agent-runtime/blob/73b057a8ab40076852018d9d032933e0f7424dd7/src/core/agents/customAgents.ts#L731

Ideally we should not do any discovery or parsing, but let Copilot do the reading.

The only problem is that for 'model' it would be good if we can do a migration from VS Code local model(s) to aCopilot model.