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/reference/configuration.md

@@ -1,7 +1,7 @@
 # Configuration
 
 Every setting comes from the environment, or from an env file. Which file
-depends on how the installation was started:
+depends on how the instance was started:
 
 | Started with | Reads |
 |---|---|
@@ -36,7 +36,7 @@ what the container images do to pin everything onto `/data`.
 | `managed` | a venv the engine builds under `DATA_DIR` and owns, which the Modules screen installs into with `uv pip sync`. The container images set this: the venv in them holds the app and nothing of anybody else's. |
 | a path | that interpreter, or that venv, whatever it is. |
 
-An installation that already has a managed venv keeps it on upgrade under
+An instance that already has a managed venv keeps it on upgrade under
 `auto`, because it may hold packages somebody installed on purpose.
 
 !!! warning "The four files that must be on persistent storage"
@@ -112,10 +112,10 @@ remember it is fixed at build time: changing it means rebuilding that image.
 `delay` with a cron expression fires on local time. Left at `UTC`, "off at
 02:00" means two in the morning UTC, which in most of the world is neither two
 o'clock nor the same hour in summer as in winter. Set it to where the
-installation is.
+instance is.
 
 `production` closes `/docs`, `/redoc` and the OpenAPI document, because the
-schema enumerates every endpoint the installation serves — including the paths
+schema enumerates every endpoint the instance serves — including the paths
 webhook nodes mounted at runtime. It also turns a `changethis` secret from a
 warning into a refusal to start.