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/base.go

@@ -6,7 +6,7 @@ type Tool interface {
 	Name() string
 	Description() string
 	Parameters() map[string]interface{}
-	Execute(ctx context.Context, args map[string]interface{}) (string, error)
+	Execute(ctx context.Context, args map[string]interface{}) *ToolResult
 }
 
 // ContextualTool is an optional interface that tools can implement