Integrate GOALS feature

[vadiminshakov]

• [+] I have read CONTRIBUTING.md.
• [-] I have created a discussion that was approved by a maintainer (for new features).

Implemented goals functionality inspired by Codex /goal

Goals allows assigning a long-term objective to an agent, which it autonomously pursues across multiple sequential turns.

• The goal is persisted in the database with an active status via goal.Service.
• After each turn, Runtime.OnTurnFinished -> MaybeContinue is triggered.
• MaybeContinue checks whether the session is idle, the queue is empty, and the goal is still active. If all conditions are met, it renders a continuation prompt from a template and launches the agent again.
• The agent can update the goal status itself through the update_goal tool (marking the goal as complete).
• In the UI, the goal is displayed as a sidebar “pill” with a timer and status indicator.