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/getting-started/facility-automation.md

@@ -53,9 +53,9 @@ $EDITOR .env                # FIRST_SUPERUSER, ENVIRONMENT=production
 | `FIRST_SUPERUSER_PASSWORD` | leave it as `changethis` and one is generated for you |
 | `ENVIRONMENT` | `production` closes the interactive API schema; `local` leaves it open |
 
-Everything the installation owns — the database, your flows, secrets,
+Everything the instance owns — the database, your flows, secrets,
 artifacts, the packages your node code imports — is on one Docker volume.
-Backing that volume up is backing up the installation.
+Backing that volume up is backing up the instance.
 
 ??? note "Even smaller: no Docker at all"
 
@@ -313,5 +313,5 @@ bad it gets. See [Secrets, modules and alerts](../interface/operations.md).
 - [The flow editor](../interface/flow-editor.md) — the canvas, in detail
 - [Keeping state in a flow](../concepts/state.md) — running totals, debounces,
   and the one rule that makes them safe
-- [Accounts and the portal](../interface/portal.md) — reach the installation
+- [Accounts and the portal](../interface/portal.md) — reach the instance
   from outside the house without opening a port