[scope] Add typed outcomes and make taskvisor's purpose clear

[mr-chelyshkin]

📝 Description

• Split attempt events from final task events.
• Add TaskOutcomeKind for reliable handling.
• Make reason strings diagnostic only.
• Enable the controller feature by default.
• Add ARCHITECTURE.md

✅ Checklist

• Documentation updated (README, API docs, examples)
• Tests added/updated (if relevant)
• task ci passes locally

Breaking

• Rename attempt events from Task* to Attempt*.
• Replace ActorExhausted and ActorDead with typed TaskFinished.
• Report force-abort through TaskFinished.outcome_kind.
• Change lifecycle event labels used by logs, metrics, and traces.