[Bug] Session fragmentation - new session file created every few messages

Bug Description

Sessions are fragmenting into dozens of separate session files instead of continuing in a single session. The session picker shows a single session with "+47 other sessions" (all with identical names), where each fragment contains only 1-5 messages.

Environment

• Platform: Windows 11
• Version: 2.1.29
• Terminal: Windows Terminal

Observed Behavior

1. Started session named "strand-rate-intelligence-2"
2. Session picker shows: "strand-rate-intelligence-2 (+47 other sessions)"
3. Expanding shows ~50 separate session entries, all named identically
4. Each fragment has only 1-5 messages
5. Timestamps are minutes apart (suggesting new file every few messages)

Expected Behavior

All messages should continue in a single session file, not fragment into separate files.

Impact

• Impossible to resume full conversation context
• Session history becomes unusable
• Each fragment loses context from previous fragments

Possibly Related

• May be related to session persistence changes
• Appears to be Windows-specific (needs confirmation)