fix: remove writeMemoryFile from session handling

[avoidwork]

Description

Removed writeMemoryFile from handleConversation() in index.js. This function was writing session files in a different format (timestamp-named files with title/timestamp frontmatter) than saveSession() (UUID-named files with threadId/startedAt frontmatter). The dual-write caused the reflectionSessions tool to find no valid sessions, since it looks for startedAt in frontmatter.

Type of Change

• Bugfix (non-breaking change which fixes an issue)

Testing

• Verified writeMemoryFile is no longer called in handleConversation()
• Confirmed saveSession() remains the sole session writer (called on shutdown)
• reflectionSessions tool expects startedAt in frontmatter — only the old format provides this

Coverage

• Line coverage maintained

Checklist

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied