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/llms.txt

@@ -5,7 +5,7 @@
 > same engine serves two shapes of work: live flows that never end (buildings,
 > labs, homelabs) and batch runs that finish and leave a record (experiments,
 > ML pipelines, CI-style jobs). Self-hosted by default; an optional portal
-> exists only to reach an installation that has no inbound route.
+> exists only to reach an instance that has no inbound route.
 
 ## Getting started
 - [Pick your starting point](/getting-started/): the two setup paths and how to choose