Release script resumes an interrupted release #158

Parent

#150

What to build

If a release is interrupted (the terminal is closed, Ctrl-C, the network drops), running the release script again with the same version carries on from the first step not yet done, with no manual cleanup:

• Branch pushed, no PR yet: it opens the PR.
• PR open: it waits for the checks, then merges and tags.
• PR merged, no tag yet: it tags that merge commit and pushes the tag.
• Tag already on origin, pointing at the merged bump: it says the release is already tagged and exits 0. This narrowly relaxes the "tag already exists" refusal.

Acceptance criteria

• A re-run after the PR was opened, but before the merge, merges and tags without opening a second PR or pushing a second bump commit.
• A re-run after the merge, but before the tag, tags the existing merge commit and pushes the tag.
• A re-run once the tag is on origin, pointing at the merged bump, reports it and exits 0 without changing anything.
• A tag that exists but doesn't point at the merged bump for this version is still refused.
• Tests go through the same black-box seam, setting up each interrupted state in the bare origin and the fake gh state.

Blocked by

• Release script: one command from version to pushed tag #156