A remote thread brings the replies that never reached us

The context of a remote post showed only what PrivaPub happened to hold:
replies from servers nobody here follows were never seen, and only the
ancestors were ever fetched. Now a persona opening a public remote thread
queues FetchReplies for the post and its root, at most once an hour each.

The job reads the thread's own collection first (FEP-7888 `context`, which
Mastodon 4.5+ serves with every reply at any depth; posts or, as FEP-f228
allows, the activities that made them), and otherwise the post's `replies`
(PeerTube's `comments`) and the replies' own, two levels down. At most 5
pages and 100 posts a job, signed by the instance actor, never a persona;
each post is fetched from its own origin and stored through StoreContext,
so only public and unlisted ones are kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

docs/INTEROP.md

@@ -147,6 +147,7 @@ Priorities, used throughout:
 | Inbound `Block`: stop delivering, hide (**done** 2026-10-04) | P2 | `relationship.blocked_by` |
 | `Add`/`Remove` featured (pins, tags) | P2 | `GET /accounts/:id/statuses?pinned=true` |
 | Remote like and boost totals | P2 | counts |
+| Read the thread's `context` collection to complete a thread (**done** 2026-10-05, `FetchReplies`) | P2 | `/statuses/:id/context` |
 | Publish `context` and a paged `replies` | P2 | — |
 | `FeatureRequest`: send `Reject` (or implement FEP-7aa9) | P3 | — |