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>

pkg/utils/string.go

@@ -0,0 +1,16 @@
+package utils
+
+// Truncate returns a truncated version of s with at most maxLen runes.
+// Handles multi-byte Unicode characters properly.
+// If the string is truncated, "..." is appended to indicate truncation.
+func Truncate(s string, maxLen int) string {
+	runes := []rune(s)
+	if len(runes) <= maxLen {
+		return s
+	}
+	// Reserve 3 chars for "..."
+	if maxLen <= 3 {
+		return string(runes[:maxLen])
+	}
+	return string(runes[:maxLen-3]) + "..."
+}