[codex] Clarify Slack skill 429 retry sequence

[ssetty-oai]

Summary

• Replace the open-ended exponential backoff wording with a bounded same-bucket retry sequence.
• Make the fallback timing explicit: about 30 seconds after the first 429, about 1 minute after the next same-bucket 429, and about 2 minutes after the next same-bucket 429.

Why

Slack tool quotas are per-minute by bucket, so the skill should give agents concrete, bounded retry guidance while still respecting Retry-After when present.

Validation

• Ran git diff --check.
• Markdown-only skill update; no automated test suite run.