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

backend/fluksio/tui/app.py

@@ -71,7 +71,7 @@ class Enroll(ModalScreen[tuple[str, str] | None]):
 
     def compose(self) -> ComposeResult:
         with Vertical(id="enroll"):
-            yield Label("Pair this installation with a portal")
+            yield Label("Pair this instance with a portal")
             yield Input(placeholder="claim code", id="code")
             yield Input(value=DEFAULT_PORTAL, id="portal")
             with Horizontal():
@@ -253,7 +253,7 @@ class ServeApp(App[int]):
             self.connect()
             return
         if who == "foreign":
-            self.note(f"Port {wanted} holds another installation's Fluksio.")
+            self.note(f"Port {wanted} holds another instance's Fluksio.")
 
         self.child = subprocess.Popen(  # noqa: S603
             child_argv(sys.argv[1:]),