feat: v2.7.8 — Artifacts hub, clone related repos as extra folders, quieter resume prompt

- Artifacts (/artifacts): claude.ai pages sessions publish (captured from the
  Artifact tool's hook), pinned workspace files, skills and plugins, each
  kept as a snapshot; shared ones appear in a Hub and a skill or plugin
  installs into the viewer's own account (server/lib/artifacts.js,
  /api/artifacts, docs/plans/artifacts/).
- Workspace extra folders: clone one or more related repos beside the
  workspace and add them in one step (POST /api/lanes/:id/sources/clone).
- A pane that ended by itself while tmux kept running is no longer offered
  for resume (tmux.serverStartedAt in reconcileOrphans).
- Changes panel: a folder that is not a git repo opens on Files, and its
  files open from there.
- Plus the parallel sessions' work in this tree: File Explorer button in the
  panel header, user delete, RSA SSH keys, chat/terminal and pet fixes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/SERVER-MODE.md

@@ -726,6 +726,21 @@ sudo agenthub server install-system
 
 Until then the File Explorer's move, copy and delete in a member's folder
 answer `409 EUPGRADE` (listing, upload, new folder/file and zip work).
+The same re-run installs `agenthub-provision delete`, behind the Users page's
+**Delete user**: until then a delete answers `502 EPROVISION` ("unknown
+action: delete") and changes nothing.
+
+### Deleting a user
+
+Users page → Edit → **Delete user** (the admin's own password, then a red
+confirm). Order: the user's logins end and their workspaces' panes stop;
+under S3 `agenthub-provision delete <name>` locks `ah-<name>`, kills its
+processes, `userdel`s it and removes `<USERS_ROOT>/<name>` (a real
+directory exactly there — never a link — `rm -rf --one-file-system`); only
+then are the rows removed (the user, their workspaces' records, prefs, push
+subscriptions, personal AI Proxy profiles). Session history and the audit log
+stay. Never yourself (`ESELF`) or the last admin (`ELASTADMIN`). A user made
+again under the same name gets a NEW, empty Linux account and home.
 
 ### What a new machine needs before `install-system`