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

docs/concepts/flows.md

@@ -144,7 +144,7 @@ Every flow has a published version and, while you are working, a draft.
 
 The store is a git repository — `flow.json` for the structure, `nodes/*.py` for
 the code — and each save is a commit. So a flow's history is readable with
-ordinary git tooling, and copying a flow between installations is copying a
+ordinary git tooling, and copying a flow between instances is copying a
 directory.
 
 Saving carries the version you last saw. If someone else saved in between, you