feat(plugin): flat tool draft with namespace

Effect draft.add accepts flat tool objects matching Promise ergonomics.
Migrate the tool grouping concept fully to namespace: rename MCP's group
helper/registration to namespace and thread namespace through the flat
Effect and Promise draft declarations.

packages/core/test/plugin/promise.test.ts

@@ -135,7 +135,7 @@ describe("fromPromise", () => {
           await ctx.tool.transform((tools) => {
             tools.add({
               name: "hello",
-              options: { codemode: false },
+              codemode: false,
               description: "Hello",
               input: Schema.Struct({ name: Schema.String }),
               output: Schema.String,