ci: Improve merge_group and branch protection

[samuelburnham]

• Fixes cycle-count-regression comparison on merge_group
• Removes redundant CI checks on push to dev, since they are already run on merge_group
• Fixes default channel for bench.yml Zulip message
• Adds a unified status check for branch protection rules called check-job-results, which will succeed if all other jobs in rust.yml succeed or are skipped (including matrix jobs). If any of them fail, then check-job-results will fail, blocking merge or dequeueing from the merge queue if it's added as a required status check in branch protection.

Successful runs:

• https://github.com/argumentcomputer/zk-light-clients/actions/runs/11633286599
• https://github.com/argumentcomputer/zk-light-clients/actions/runs/11633718291
• https://github.com/argumentcomputer/zk-light-clients/actions/runs/11633939094

[huitseeker]

thanks!