feat: parallelize bakery ci publish across image targets

[ianpittwood]

Summary

bakery ci publish ran its 4-phase pipeline (oras index-create → soci convert → oras index-copy → verify) serially per target, with no retry — so transient GHCR eventual-consistency failures (#591) failed the whole job.

This tracking issue documents that work, implemented across:

• Merge soci and oras plugins into imagetools plugin #619 — merge soci + oras plugins into a single imagetools plugin
• feat: parallelize bakery ci publish across image targets #641 — parallelize publish across image targets via ShellJob/run_jobs/CommandRunner (extending posit_bakery/parallel/ from feat: parallel dgoss execution via shared ParallelShellExecutor #588)
• feat: retry-with-backoff for the parallel publish path #642 — retry-with-backoff for the parallel publish path

Filed retroactively for visibility/history alongside the sibling parallelization issues; no additional implementation is expected here beyond what's in the linked PRs.

Part of the investigation started in #584.