Clarify executeTool input and schema values

[mlmrx]

Summary

• clarify that executeTool() takes a RegisteredTool returned by getTools()
• clarify that RegisteredTool.inputSchema is exposed as a parsed JavaScript object
• clarify that callers pass the structured JavaScript input value directly rather than pre-serializing it with JSON.stringify()
• add a focused discovery-and-execution example to the API introduction

This documents the contract already defined by the RegisteredTool dictionary and the
executeTool() serialization algorithm. An implementation that exposes inputSchema as a string or
requires pre-serialized JSON arguments does not match the current draft.

Closes #278.

Validation

• W3C spec-generator: Bikeshed processing with die-on=warning
• git diff --check