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

.env.example

@@ -36,7 +36,7 @@ SMTP_TLS=True
 SMTP_SSL=False
 SMTP_PORT=587
 
-# Where this installation keeps everything: the database, flows, secrets,
+# Where this instance keeps everything: the database, flows, secrets,
 # artifacts and the user venv. The compose stack sets it to the /data volume.
 DATA_DIR=flow-data
 # Any SQLAlchemy URL. Empty means SQLite in the data directory, which is what