ci(proofs): improve how slack notifications on failure are delivered

In https://github.com/ethereum-optimism/optimism/pull/11015, we offloaded the kontrol proofs compute from CircleCI to github runners hosted by Runtime Verification (RV).

Ideally, the slack notification would be triggered by a workflow that lives within this monorepo so we have control over notification delivery. However, the current architecture does not support this easily—we initially tried using [slack-github-action](https://github.com/slackapi/slack-github-action) with an `on: status` trigger in the workflow, but *every* commit reports a status so this flooded our github actions.

Instead, we provided a slack webhook URL to RV (used [here](https://github.com/runtimeverification/optimism-ci/blob/0d4572bede9e83fbca81587900923c7039484e5b/.github/workflows/optimism-ci.yaml#L184)) which posts the failure notification.

RV is working on improvements to their Kontrol as a Service setup, so around Q4 this year we should be able to update this approach so the slack posting happens from within the monorepo

cc @raffaele-oplabs @F-WRunTime

Start by reading the current notification workflow at runtimeverification/optimism-ci/.github/workflows/optimism-ci.yaml around line 184 and the architecture described in PR #11015. Confirm when Kontrol as a Service supports moving Slack posting into this monorepo; done means failures notify Slack from a repository workflow without triggering notifications for every commit.

Written by the indexing model from the issue text.