wip: P1 feature batch — retry, search, code symbols, diff/status/test commands, tool output head+tail

- retry logic for transient LLM errors (openai.go)
- file_search context_lines + max_results (file.go)
- FormatResult now preserves head+tail with omitted bytes (tools.go)
- /status /diff /test slash commands wired (workflow.go, app.go)
- CodeSymbolsTool for Go package/type/func listing (code.go)
- README updated with v0.0.4 features

cmd/agentu/main.go

@@ -114,6 +114,7 @@ func run() error {
 			Yolo:         *yolo,
 			ThemeMode:    themeMode,
 			ModelManager: modelManager,
+			WorkingDir:   cfg.Agent.WorkingDir,
 		})
 		printExitSession(modelManager)
 		return err