Convergence & migration (reviewbot → gittensory)

[JSONbored]

Part of #975.

Context

Merge the maintainer-automation into gittensory and migrate the live hosted service onto the converged stack — without downtime or review gaps. Convergence proceeds strangler-fig, per-repo, only on an explicit signal.

Deliverables

• The merge plan (code + infra) for bringing the automation core into gittensory.
• Per-repo cutover on an explicit signal; the legacy hosted path stays live until each repo is verified.
• Parity verification — same outcomes pre/post cutover on a real sample — plus a rollback path per repo.
• Decommission the legacy path once all repos are migrated and verified.
• Self-host getting-started docs (requirements, install, configure, choose an AI backend, run) published alongside.

Acceptance criteria

• Each repo migrates with verified parity and a tested rollback.
• No review/automation gaps occur during migration; the hosted service is never down.

Notes

• Sequence + timing are owner-driven; this issue tracks the mechanism, not a fixed date.