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_runtime_control.py

@@ -237,7 +237,7 @@ def lifecycle(monkeypatch: pytest.MonkeyPatch) -> list[str]:
 def test_rebuilding_one_flow_leaves_another_flows_node_running(
     tmp_path: Path, lifecycle: list[str]
 ):
-    """The whole point: reconnecting one flow's nodes, not the installation's."""
+    """The whole point: reconnecting one flow's nodes, not the instance's."""
     store = FlowStore(tmp_path / "flows")
     for name in ("a", "b"):
         store.write_flow(FlowDef(name=name, nodes=[NodeDef(id="io", type="lifecycle")]))