Add persistent AI cache for TLDRs and chat with 7-day expiry

[brianlovin]

Summary

Implements persistent caching for AI-generated content (TLDRs and chat sessions) that survives application restarts. These caches are stored in the user's home directory with a 7-day expiration policy, while the existing session cache remains in the temp directory.

Key Changes

• Persistent cache storage: TLDR and chat caches now stored in ~/.cache/hn-cli/ instead of temp directory, allowing them to persist across reboots
• 7-day expiration: Both TLDR and chat caches expire after 7 days, with automatic cleanup of stale entries on load/save
• Cache merging: Session cache and persistent cache are intelligently merged during initialization, with persistent cache taking precedence for existing entries
• Immediate persistence: TLDR cache is saved immediately after generation completes, not just on app exit
• Graceful degradation: All cache operations fail silently to prevent crashes if filesystem operations fail

Implementation Details

• Added loadTLDRCache() and saveTLDRCache() functions to manage TLDR persistence
• Added loadChatCache() and saveChatCache() functions to manage chat session persistence
• New cache interfaces: CachedTLDR, PersistentChatSession, TLDRCache, and ChatCache with timestamp tracking
• Cache entries are keyed by story ID as strings for JSON serialization
• Existing timestamps are preserved when re-saving caches to maintain accurate expiration times
• Expired entries are filtered out during load and save operations