CODEX_SESSION_SCANNER was incorrectly auto-resuming old sessions because
session files persist after Codex exits. Fixed by:
1. Only matching sessions created AFTER hapi startup (reject if
sessionTimestamp < referenceTimestampMs)
2. Snapshot resumeSessionId at startup so scanner callbacks don't
affect codexLocal's launch parameters
3. Updated session timestamp test to verify boundary conditions
Reorganized runCodex.ts to improve maintainability by extracting:
- CodexSession class for session lifecycle management
- CodexLocalLauncher and CodexRemoteLauncher for mode-specific initialization
- CodexEventConverter for MCP message handling and UI buffer updates
- CodexSessionScanner for resume file discovery
- emitReadyIfIdle utility for ready event emission
Added codexSessionId field to metadata schema for session tracking.
Updated UI components to work with refactored architecture.
