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/flow/modules.py

@@ -97,7 +97,7 @@ def venv_dir() -> Path:
 def venv_python() -> str:
     """The interpreter node code runs on.
 
-    Falls back to the engine's own when there is no venv — an installation that
+    Falls back to the engine's own when there is no venv — an instance that
     could not build one still runs python nodes, it just cannot add packages
     to them.
     """