refactor(agent): improve code quality and restore summarization

Code review fixes:
- Use map for O(n) job lookup in cron service (was O(n²))
- Set DeleteAfterRun=true for one-time cron tasks
- Restore context compression/summarization to prevent context overflow
- Add pkg/utils/string.go with Unicode-aware Truncate function
- Simplify setupCronTool to return only CronService
- Change Chinese comments to English in context.go

Refactoring:
- Replace toolsSummary callback with SetToolsRegistry setter pattern
- This makes dependency injection clearer and easier to test

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

cmd/picoclaw/main.go

@@ -552,7 +552,7 @@ func gatewayCmd() {
 		})
 
 	// Setup cron tool and service
-	cronService, _ := setupCronTool(agentLoop, msgBus, cfg.WorkspacePath())
+	cronService := setupCronTool(agentLoop, msgBus, cfg.WorkspacePath())
 
 	heartbeatService := heartbeat.NewHeartbeatService(
 		cfg.WorkspacePath(),
@@ -690,7 +690,7 @@ func getConfigPath() string {
 	return filepath.Join(home, ".picoclaw", "config.json")
 }
 
-func setupCronTool(agentLoop *agent.AgentLoop, msgBus *bus.MessageBus, workspace string) (*cron.CronService, *tools.CronTool) {
+func setupCronTool(agentLoop *agent.AgentLoop, msgBus *bus.MessageBus, workspace string) *cron.CronService {
 	cronStorePath := filepath.Join(workspace, "cron", "jobs.json")
 
 	// Create cron service
@@ -706,7 +706,7 @@ func setupCronTool(agentLoop *agent.AgentLoop, msgBus *bus.MessageBus, workspace
 		return result, nil
 	})
 
-	return cronService, cronTool
+	return cronService
 }
 
 func loadConfig() (*config.Config, error) {