streaming tools protocol: native ToolCall format, ToolsStream, StreamingChunk.ToolCalls

[JaimeStill]

Summary

• Refactors ToolCall from flat canonical format with custom MarshalJSON/UnmarshalJSON to native LLM API format ({id, type, function: {name, arguments}}), aligning with OpenAI-compatible wire format
• Adds ToolFunction named type and NewToolCall constructor
• Extends StreamingChunk.Delta with ToolCalls field and ToolCalls() accessor
• Adds ToolsStream to Agent interface, implementation, and mock
• Embeds protocol.ToolCall in kernel's ToolCallRecord, eliminating field duplication
• Adds NewStreamingToolsAgent mock helper

Closes #23

Test plan

• All existing tests updated for native ToolCall format
• New tests: TestNewToolCall, TestToolCall_Marshal_NativeFormat, TestToolCall_Marshal_RoundTrip
• New tests: TestStreamingChunk_ToolCalls, TestStreamingChunk_ToolCalls_ContinuationChunk, TestParseToolsStreamChunk
• New tests: TestMockAgent_ToolsStream, TestNewStreamingToolsAgent
• go test ./... passes
• go vet ./... passes