feat: US-002 - Modify Tool interface to return *ToolResult

- Update all Tool implementations to return *ToolResult instead of (string, error)
- ShellTool: returns UserResult for command output, ErrorResult for failures
- SpawnTool: returns NewToolResult on success, ErrorResult on failure
- WebTool: returns ToolResult with ForUser=content, ForLLM=summary
- EditTool: returns SilentResult for silent edits, ErrorResult on failure
- FilesystemTool: returns SilentResult/NewToolResult for operations, ErrorResult on failure
- Temporarily disable cronTool in main.go (will be re-enabled in US-016)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/tools/result.go

@@ -0,0 +1,143 @@
+package tools
+
+import "encoding/json"
+
+// ToolResult represents the structured return value from tool execution.
+// It provides clear semantics for different types of results and supports
+// async operations, user-facing messages, and error handling.
+type ToolResult struct {
+	// ForLLM is the content sent to the LLM for context.
+	// Required for all results.
+	ForLLM string `json:"for_llm"`
+
+	// ForUser is the content sent directly to the user.
+	// If empty, no user message is sent.
+	// Silent=true overrides this field.
+	ForUser string `json:"for_user,omitempty"`
+
+	// Silent suppresses sending any message to the user.
+	// When true, ForUser is ignored even if set.
+	Silent bool `json:"silent"`
+
+	// IsError indicates whether the tool execution failed.
+	// When true, the result should be treated as an error.
+	IsError bool `json:"is_error"`
+
+	// Async indicates whether the tool is running asynchronously.
+	// When true, the tool will complete later and notify via callback.
+	Async bool `json:"async"`
+
+	// Err is the underlying error (not JSON serialized).
+	// Used for internal error handling and logging.
+	Err error `json:"-"`
+}
+
+// NewToolResult creates a basic ToolResult with content for the LLM.
+// Use this when you need a simple result with default behavior.
+//
+// Example:
+//
+//	result := NewToolResult("File updated successfully")
+func NewToolResult(forLLM string) *ToolResult {
+	return &ToolResult{
+		ForLLM: forLLM,
+	}
+}
+
+// SilentResult creates a ToolResult that is silent (no user message).
+// The content is only sent to the LLM for context.
+//
+// Use this for operations that should not spam the user, such as:
+// - File reads/writes
+// - Status updates
+// - Background operations
+//
+// Example:
+//
+//	result := SilentResult("Config file saved")
+func SilentResult(forLLM string) *ToolResult {
+	return &ToolResult{
+		ForLLM:  forLLM,
+		Silent:  true,
+		IsError: false,
+		Async:   false,
+	}
+}
+
+// AsyncResult creates a ToolResult for async operations.
+// The task will run in the background and complete later.
+//
+// Use this for long-running operations like:
+// - Subagent spawns
+// - Background processing
+// - External API calls with callbacks
+//
+// Example:
+//
+//	result := AsyncResult("Subagent spawned, will report back")
+func AsyncResult(forLLM string) *ToolResult {
+	return &ToolResult{
+		ForLLM:  forLLM,
+		Silent:  false,
+		IsError: false,
+		Async:   true,
+	}
+}
+
+// ErrorResult creates a ToolResult representing an error.
+// Sets IsError=true and includes the error message.
+//
+// Example:
+//
+//	result := ErrorResult("Failed to connect to database: connection refused")
+func ErrorResult(message string) *ToolResult {
+	return &ToolResult{
+		ForLLM:  message,
+		Silent:  false,
+		IsError: true,
+		Async:   false,
+	}
+}
+
+// UserResult creates a ToolResult with content for both LLM and user.
+// Both ForLLM and ForUser are set to the same content.
+//
+// Use this when the user needs to see the result directly:
+// - Command execution output
+// - Fetched web content
+// - Query results
+//
+// Example:
+//
+//	result := UserResult("Total files found: 42")
+func UserResult(content string) *ToolResult {
+	return &ToolResult{
+		ForLLM:  content,
+		ForUser: content,
+		Silent:  false,
+		IsError: false,
+		Async:   false,
+	}
+}
+
+// MarshalJSON implements custom JSON serialization.
+// The Err field is excluded from JSON output via the json:"-" tag.
+func (tr *ToolResult) MarshalJSON() ([]byte, error) {
+	type Alias ToolResult
+	return json.Marshal(&struct {
+		*Alias
+	}{
+		Alias: (*Alias)(tr),
+	})
+}
+
+// WithError sets the Err field and returns the result for chaining.
+// This preserves the error for logging while keeping it out of JSON.
+//
+// Example:
+//
+//	result := ErrorResult("Operation failed").WithError(err)
+func (tr *ToolResult) WithError(err error) *ToolResult {
+	tr.Err = err
+	return tr
+}