docs: update Cuppy long-task runtime - #203

What changed

• Update the AI Chat long-task guidance in English and Chinese.
• Document the current approximate 24-hour conversation environment lifetime and unchanged 30-minute idle release behavior.
• Reframe checkpoint saves as protection for multi-stage work instead of requiring long tasks to be split by default.
• Remove the outdated long-task capability note.

Why

The Help Center still described a 5-hour maximum lifetime. Cuppy now provides an approximate 24-hour conversation environment, which is sufficient for most long-running analysis, file processing, and app-building tasks.

Validation

• English and Chinese punctuation checks
• git diff --check
• mintlify validate