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/flow/test_provision.py

@@ -192,7 +192,7 @@ def test_a_node_waiting_for_a_machine_asks_for_one_once(monkeypatch):
     assert ran.is_set()
 
 
-def test_an_installation_with_nowhere_to_start_one_has_no_file(tmp_path):
+def test_an_instance_with_nowhere_to_start_one_has_no_file(tmp_path):
     assert load_provisioners(tmp_path / "provisioners.json") == []