Decide whether to wire CostGuard soft limits into the runtime path

[simonhsze]

What

Decide whether CostGuard.check_soft_limit() should remain a library helper only or become part of the actual agent runtime flow.

Motivation

CostGuard currently exposes a soft-limit signal, but the method is only exercised in tests and is not used by BaseAgent or hook flow. That leaves an awkward middle state: the API suggests a warning path exists, but the runtime currently only enforces post-call hard-limit behavior.

Scope

• decide whether soft limits should be part of the main agent lifecycle
• if yes, add a clear hook or warning path for soft-limit crossings
• if no, simplify the API or document that soft-limit checks are library-level utilities only
• update tests and docs accordingly

Why this matters

This is a small but important engineering signal. Public APIs should either be part of the real flow or clearly marked as optional helpers, not left in an ambiguous in-between state.