[release tool]: merge staging pull requests through the tool itself

Currently it is unclear what should be used to merge staging pull requests. We should just let the release tool merge the PR as it knows exactly where the PR should go into. The merge tool would rely on target labels which might not work temporarily e.g. when branching-off a feature freeze.

We should either merge automatically once the PR is approved + checks are passing, or we just add a double-prompt that can be submitted to manually start the merging (useful in case the PR checks are not significant as in COMP and FW - though worth noting that this could be done through a configuration as well in `.ng-dev`.)

The cherry-pick PRs should be merged automatically based on our discussion

Start by locating the release tool's staging pull-request merge flow and reviewing the `.ng-dev` configuration mentioned in the issue. Compare automatic merging with the proposed double-prompt and configuration options, including the stated cherry-pick behavior. Done means the merge policy is decided and implemented for the described staging and cherry-pick cases.

Written by the indexing model from the issue text.