writ repo remove leaves the clone it made in the writ-owned home #792

Feature: #463 — writ command surface: layers, segments, adoption.

writ repo add <layer> <repository-url> clones into the writ-owned home, XDG_DATA_HOME/devlore/writ/repos/<layer>.
writ repo remove <layer> drops the registration and leaves that clone behind. The next add of the same
layer from a URL then finds the directory occupied, and the user is cleaning up writ’s own state by hand.

Ruled 2026-09-04: a clone writ made in its own home is writ’s to remove. Unregistering a layer removes
the registration and the writ-owned clone, and says so. A working tree the user registered by path — theirs,
anywhere else on disk — is never touched; the registration alone goes. The same distinction governs set
replacing a registration (#791): the displaced writ-owned clone is removed, a displaced user path is left
alone.

The verbs become set and unset in #791; this fix rides on whichever spelling lands first. Both are lanes of
#887 (schedule) — #791 is lane 8, this is lane 9.

The record, ruled 2026-09-18

Ownership is not narration: it reaches stdout as a field, so a script and a --dry-run can both read it. The
shape is #791’s, and unset emits:

{
  "layer": "personal",
  "state": "unregistered",
  "previous": {
    "source": "[email protected]:me/personal.git",
    "root": "/Users/me/.local/share/devlore/writ/repos/personal",
    "owner": "writ"
  }
}

previous.owner is the whole ruling: writ means the tree at previous.root was removed with the
registration, user means only the registration went. There is no separate removed field — writ clones only
into <data home>/devlore/writ/repos/<name>, so a removed clone is always previous.root.

--dry-run emits the same record and does nothing, which is how a caller learns what would go.

Acceptance

• unsetting a layer registered from a URL removes XDG_DATA_HOME/devlore/writ/repos/<layer> and narrates it
• unsetting a layer registered by path removes the registration only, asserted
• the record carries previous.root and previous.owner, and --dry-run emits it without removing anything
• the guide docs/guides/writ/repositories.md says which clones writ owns and removes