Add tool result streaming for long-running operations

[tombee]

Problem

When tools execute long-running operations (shell commands, file processing), the agent and UI receive no feedback until the operation completes. This creates poor UX for:

• Build commands that take minutes
• Large file operations
• Network requests with slow responses

Concrete example from foreman: Running npm test during implementation can take 30+ seconds. Users see no progress until completion.

Proposal

Add streaming support for tool results:

// Tool interface extension
type StreamingTool interface {
    Tool
    ExecuteStream(ctx context.Context, inputs map[string]any) (<-chan ToolChunk, error)
}

type ToolChunk struct {
    Data     string
    IsFinal  bool
    Metadata map[string]any
}

// SDK emits events as chunks arrive
s.OnEvent(sdk.EventToolOutput, func(e *sdk.Event) {
    chunk := e.Data.(*sdk.ToolOutputEvent)
    // Stream to UI
})

General Utility

1. CI/CD agents - Build and test output streaming
2. Data processing - Progress indicators for large operations
3. Interactive shells - Real-time command output

Acceptance Criteria

• StreamingTool interface for tools that support streaming
• EventToolOutput event type for streaming chunks
• Built-in shell tool supports streaming stdout/stderr
• Non-streaming tools continue working unchanged
• Integration test demonstrating streaming behavior

Priority

Medium - Improves UX but not blocking for MVP.

Labels

enhancement, sdk, priority:medium

[tombee]

Implemented in PR #13 (SPEC-5)