Fix: session context persistence

[michaelansel]

Fixed two critical issues causing conversation context to reset on every message:

1. Session continuation detection: Replaced unreliable is_new_session flag with robust temp_ prefix check

   • The is_new_session flag persisted incorrectly in cached session objects
   • Now checks if session_id starts with "temp_" to determine if it's a new session
   • Ensures --resume flag is properly used for continuing conversations

2. Backend consistency: Added backend tracking to prevent SDK/subprocess session conflicts

   • SDK and subprocess have separate session storage and cannot resume each other's sessions
   • Added backend field to ClaudeSession to track which backend created each session
   • Sessions now stick with their original backend (SDK or subprocess) for their lifetime
   • Prevents context loss when SDK fails and falls back to subprocess

Before this fix:

• First message: Creates session (subprocess fallback due to SDK error)
• Second message: SDK tries to resume subprocess session → fails → context lost

After this fix:

• First message: Creates session with backend='subprocess'
• Second message: Uses subprocess directly → context maintained ✓

🤖 Generated with Claude Code

[michaelansel]

I ran into what seems like the same problems as #6 , but Claude came up with a different solution. I think it might be worth merging the two, so I'm submitting this now as an interim hold until I have time to do the integration/comparison.