chore: add 7-day Dependabot cooldown

[theorderingmachine]

Adds a 7-day Dependabot cooldown to the npm and github-actions update blocks in .github/dependabot.yml, while excluding @slack/* packages from the npm cooldown so Slack-owned packages can still update immediately.

This is partly motivated by prior workflow hardening / analysis work around zizmor in this repo and related health-score-driven PRs:

• zizmor docs: https://docs.zizmor.sh/
• zizmor repo: https://github.com/zizmorcore/zizmor
• prior repo PR: ci: pin action hashes and escape variables with minimum permission #441
• health score / secure practices PR: ci: check for correct spellings and secure practices in actions #484

The goal here is to slow down most ecosystem updates a bit, without getting in the way of first-party @slack/* package updates or adjacent workflow security work.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @openclaw to sign the Salesforce Inc. Contributor License Agreement.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @theorderingmachine to sign the Salesforce Inc. Contributor License Agreement.