Refactor provider pace capability into descriptors

[steipete]

Summary

• Move reset-window pace eligibility and inferred monthly duration from the menu model's provider switch into ProviderDescriptor capabilities.
• Declare the existing Copilot, Cursor, Grok, Alibaba, Doubao, and OpenCode Go rules beside their provider descriptors.
• Add an all-provider equivalence test that compares the descriptor-driven behavior with the removed legacy mapping.
• Stabilize two environment-sensitive tests exposed by the full suite: PTY overflow under host load and localized ZenMux expiry dates.

Testing

• make check
• swift test --filter ProviderPaceCapabilityTests
• swift test --filter BoundedChildProcessProofTests
• swift test --filter ZenMuxProviderTests
• make test — 61/61 groups passed on the first attempt, with zero retries or timeouts
• structured autoreview — no accepted or actionable findings