go_invoke_tool does json.Marshal(r) on handler results. When r is a string,
the raw JSON gets wrapped in quotes and escapes, causing the core's
json.Unmarshal into map[string]interface{} to fail (displayed as map[]).
Changed return type to json.RawMessage (implements json.Marshaler, outputs
raw bytes inline). Added rawString() helper for callers that need the
string representation.
- handleChannelOutput: read payload/meta/type instead of content JSON
- Route by type: text/voice/image/file
- buildOutputHelp: show new parameter format
- Remove fixJSON (no longer needed without JSON-within-JSON)
