cli+webhook: look up actual base branch from GitHub API instead of hardcoding main

The CLI was hardcoding base_branch='main' causing failures on repos where
the default branch is master or another name. Now both the CLI submit
command and webhook enqueueing look up the actual base branch via
'gh api repos/{owner}/{repo}/pulls/{pr} --jq .base.ref' and store
the correct value.

Author: detain