docs: use fluent tool builder syntax in README

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Summary

• Replace verbose JSON schema construction with cleaner fluent API examples
• Lead with ToolBuilder for full control, then make_tool() for quick one-liners
• Auto-generated JSON schemas from C++ types - no more manual JSON!

Before

calc_tool.parameters_schema = copilot::json{
    {"type", "object"},
    {"properties", {{"expression", {{"type", "string"}}}}},
    {"required", {"expression"}}
};

After

auto calc = copilot::ToolBuilder("calculator", "Perform math calculations")
    .param<std::string>("expression", "Math expression to evaluate")
    .handler([](std::string expression) { return "42"; });

Test plan

• README snippets verified to compile and run