Make the source repositories read-only

Because history is not carried across, the source repositories become the only copy of each
host's history. They are kept read-only permanently rather than deleted, so that
git log and git blame remain answerable and old commit references keep resolving.

Their open pull requests must be drained first, since making a repository read-only strands
whatever is in flight. Without history preservation those can simply be re-created here as
ordinary pull requests, with no rebasing onto rewritten history.

Depends on: #685.

Steps

• Drain or re-create the open pull requests in each source repository
• Make each source repository read-only
• Confirm each import commit records its source commit

Done when

No source repository accepts writes, none has been deleted, and every imported tree traces
back to a recorded commit.