fix(boot): resolve built-in provider pricing safely

[taibai233]

Summary

Splits the provider pricing fallback out of the Guardian Subagent work into a standalone PR.

• Adds resolvePricing for boot-time agent/task/skill/planner pricing wiring.
• Preserves explicitly configured provider prices.
• Falls back only when the provider identity matches a built-in provider (name, kind, base_url) and the built-in provider serves the model.
• Avoids assigning built-in pricing to third-party providers that happen to expose the same model name.

Why

The Guardian PR introduced pricing fallback while wiring Guardian usage display, but this affects executor, planner, task, and skill pricing globally. Keeping it separate makes the behavior easier to review and avoids mixing it with Guardian approval semantics.

Validation

• go test ./internal/boot -run TestResolvePricing