Add account switch usage foundation

[cbusillo]

Summary

• add a persisted account usage store for token totals, rate-limit snapshots, usage-limit hints, and refresh-attempt gating
• add account-switch candidate selection with tried-account tracking and optional API-key fallback
• expose the core modules for the next runtime/TUI integration pass

Audit alignment

This repairs the next auth-cluster audit miss after the good #166-#171 passes: Codex Lab had no Every Code-style account usage/account switching foundation at all. This PR lands the storage and selection layer first so the follow-up runtime hooks can be small and testable.

Good PRs passed before this miss: #166, #167, #168, #169, #170, #171.

Validation

• cargo fmt -p codex-core -- --check
• git diff --check
• scoped review agents: initial findings fixed, re-review reported no blocking regressions
• attempted focused cargo test/check, but local codex-core compile exceeded the session timeout; leaving full validation to CI