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/tests/test_cli.py

@@ -106,7 +106,7 @@ def test_a_sweeps_param_spelling_is_refused_by_name() -> None:
         _params(definition, ["--lr", "fast"])
 
 
-def test_serve_uses_the_installation_the_directory_belongs_to(
+def test_serve_uses_the_instance_the_directory_belongs_to(
     tmp_path: Path, monkeypatch
 ) -> None:
     """A repository with its own venv gets its own engine, not the machine's."""
@@ -713,7 +713,7 @@ def test_who_holds_the_port_is_told_apart_by_the_token() -> None:
 
     The token is signed with this directory's secret key, so an engine that
     accepts it is one reading this directory's database. Another
-    installation's Fluksio answers the health check and refuses it.
+    instance's Fluksio answers the health check and refuses it.
     """
     import httpx