refactor(index): integrate session-state module and remove local variables
- Add imports: agent-loader, mcp-loader, session-state, logger
- Remove local session variables (mainSessionID, currentSessionID, currentSessionTitle)
- Use setter/getter functions from session-state module
- Add agent loading in config hook (loadUserAgents, loadProjectAgents)
- Add MCP loading in config hook (loadMcpConfigs)
- Replace console.error with logger
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