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/filesystem.go

@@ -30,18 +30,18 @@ func (t *ReadFileTool) Parameters() map[string]interface{} {
 	}
 }
 
-func (t *ReadFileTool) Execute(ctx context.Context, args map[string]interface{}) (string, error) {
+func (t *ReadFileTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
 	path, ok := args["path"].(string)
 	if !ok {
-		return "", fmt.Errorf("path is required")
+		return ErrorResult("path is required")
 	}
 
 	content, err := os.ReadFile(path)
 	if err != nil {
-		return "", fmt.Errorf("failed to read file: %w", err)
+		return ErrorResult(fmt.Sprintf("failed to read file: %v", err))
 	}
 
-	return string(content), nil
+	return NewToolResult(string(content))
 }
 
 type WriteFileTool struct{}
@@ -71,27 +71,27 @@ func (t *WriteFileTool) Parameters() map[string]interface{} {
 	}
 }
 
-func (t *WriteFileTool) Execute(ctx context.Context, args map[string]interface{}) (string, error) {
+func (t *WriteFileTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
 	path, ok := args["path"].(string)
 	if !ok {
-		return "", fmt.Errorf("path is required")
+		return ErrorResult("path is required")
 	}
 
 	content, ok := args["content"].(string)
 	if !ok {
-		return "", fmt.Errorf("content is required")
+		return ErrorResult("content is required")
 	}
 
 	dir := filepath.Dir(path)
 	if err := os.MkdirAll(dir, 0755); err != nil {
-		return "", fmt.Errorf("failed to create directory: %w", err)
+		return ErrorResult(fmt.Sprintf("failed to create directory: %v", err))
 	}
 
 	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
-		return "", fmt.Errorf("failed to write file: %w", err)
+		return ErrorResult(fmt.Sprintf("failed to write file: %v", err))
 	}
 
-	return "File written successfully", nil
+	return SilentResult(fmt.Sprintf("File written: %s", path))
 }
 
 type ListDirTool struct{}
@@ -117,7 +117,7 @@ func (t *ListDirTool) Parameters() map[string]interface{} {
 	}
 }
 
-func (t *ListDirTool) Execute(ctx context.Context, args map[string]interface{}) (string, error) {
+func (t *ListDirTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
 	path, ok := args["path"].(string)
 	if !ok {
 		path = "."
@@ -125,7 +125,7 @@ func (t *ListDirTool) Execute(ctx context.Context, args map[string]interface{})
 
 	entries, err := os.ReadDir(path)
 	if err != nil {
-		return "", fmt.Errorf("failed to read directory: %w", err)
+		return ErrorResult(fmt.Sprintf("failed to read directory: %v", err))
 	}
 
 	result := ""
@@ -137,5 +137,5 @@ func (t *ListDirTool) Execute(ctx context.Context, args map[string]interface{})
 		}
 	}
 
-	return result, nil
+	return NewToolResult(result)
 }