fix(agent-gui): retain active session synchronization

[rv4no]

Summary

• retain the host synchronization lease for the exact focused AgentGUI Session
• release it on switch, clear, or controller disposal
• report synchronization failures through the existing runtime diagnostics path
• document the stale planning symptom and ownership boundary

Validation

• pnpm --filter @tutti-os/agent-gui exec vitest run agentConversationMessageController.spec.ts
• pnpm check:changed -- --push-ready (12 lanes passed)

Review

• Architecture: shared focused-conversation controller owns one optional lease; the host remains transport owner
• Performance: repeated selection is idempotent and only one focused Session lease is retained
• Consumers: capability remains optional for hosts that do not expose session synchronization
• Windows: platform-neutral controller logic; no filesystem, process, or path behavior changed

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