Rename Installation to Instance

Follows the portal: the noun is "instance" everywhere the app says it —
UI strings, CLI output, error details, docs and comments. The wire keys
(`instance_id`, `instance_token`) and the hub route this calls move with it.

An existing cloud.json is adopted rather than refused: without the key
alias the dataclass fails to parse, which the caller swallows and reads as
"never enrolled" instead of "reconnect".

`instance_key` on a node type becomes `target_key`. It means the outside
thing a node points at, which is a different sense of the word, and keeping
both would put two meanings of "instance" in one codebase.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015YrQnKV3bnQd4K342y8tKj

frontend/src/components/Common/DashboardMosaic.tsx

@@ -19,8 +19,8 @@ const DEFAULT_COLUMNS = 12
  * How many tiles are worth a request of their own.
  *
  * ponytail: the list endpoint carries no placements, so a footprint means
- * reading that dashboard's document — cheap for the handful an installation
- * has, and shared with the editor's own cache. The ceiling is an installation
+ * reading that dashboard's document — cheap for the handful an instance
+ * has, and shared with the editor's own cache. The ceiling is an instance
  * with dozens: the tiles past this show their name and nothing else, and the
  * fix would be a stored footprint on `DashboardSummary`.
  */