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/index.html

@@ -5,9 +5,9 @@
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="theme-color" content="#59849b" />
-    <!-- `%BASE_URL%` is `/` on an installation of its own and `/app-shell/` in
+    <!-- `%BASE_URL%` is `/` on an instance of its own and `/app-shell/` in
          the build the portal serves, where the hub rewrites this line to the
-         installation's own path so the install is scoped to one house. -->
+         instance's own path so the install is scoped to one house. -->
     <link rel="manifest" href="%BASE_URL%manifest.webmanifest" />
     <link rel="apple-touch-icon" href="%BASE_URL%apple-touch-icon.png" />
     <title>Fluksio</title>