fix: support concurrent documentation deploys to GitHub Pages

When multiple release/snapshot builds publish documentation at the same
time, their pushes to the documentation branch race. Previously a rejected
push triggered a plain `git pull --rebase`, which stops on a merge conflict
in the shared folders (latest, X.X.x, index.html, snapshot) and, under
`set -e`, kills the whole job - so concurrent releases fail.

Make the retry loop concurrency-safe:

- Rebase the deploy commit onto the remote with `-X theirs` so shared-folder
  file conflicts auto-resolve in favour of this deploy, and add bounded
  backoff with jitter between attempts.
- Reconcile owned paths after the rebase: purge-owned folders are restored
  wholesale from the deploy commit (clean last-writer-wins) while merge-mode
  folders and other deploys' independent version folders are preserved.
- Keep `latest` as highest-version-wins: a lower release replayed after a
  concurrent higher release yields `latest` back to the remote instead of
  overwriting it with older docs.
- Resolve modify/delete rebase conflicts that `-X theirs` cannot handle
  (a file this deploy purges but a concurrent deploy modified) by driving
  the rebase to completion deterministically instead of aborting.

Add a Testcontainers regression test that lands a concurrent deploy
modifying a purged file and asserts the retry recovers cleanly.

Assisted-by: claude-code:claude-4.8-opus