[codex] Fix Freebuff 429 error messaging

[brandonkachen]

Summary

Fixes Freebuff's user-facing handling for untyped/provider-style HTTP 429 errors so users are prompted to retry instead of seeing provider or billing-oriented wording about credits/payment.

Details

• Adds shared CLI helpers for 429 detection and typed free_mode_rate_limited detection.
• Shows a Freebuff-only generic retry message for untyped 429s after waiting-room and Freebuff quota errors have had a chance to handle their own messages.
• Preserves typed Freebuff quota messages, which can include useful retry timing.
• Updates the error schema docs and regression tests.

Validation

• bun test cli/src/utils/__tests__/error-handling.test.ts
• bun cli/scripts/prebuild-agents.ts && bun test cli/src/hooks/helpers/__tests__/send-message.test.ts
• git diff --check