runtime: guard transfer_task against cyclic and excessive nested delegation

[Sayt-0]

Summary

Nested transfer_task delegation currently has no cycle detection or explicit depth limit. A cyclic configuration such as a -> b -> a can continue delegating indefinitely.

Expected behavior

Cyclic delegation should be rejected, and nested delegation should have a documented maximum depth or another explicit termination guard.

Acceptance criteria

• Direct and indirect delegation cycles are rejected.
• Valid acyclic multi-level delegation remains supported.
• Excessive nesting stops with an actionable error.
• Tests cover direct cycles, indirect cycles, and the configured depth boundary.
• Foreground and background delegation paths are covered.