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/code/agents.md

@@ -106,6 +106,6 @@ you what the canvas would have told you.
 
 - MCP is not currently reachable through a [portal](../interface/portal.md)
   tunnel — the proxy forwards `/api/v1/` only. Connect an agent on the same
-  network as the installation.
+  network as the instance.
 - Secrets are never readable, by an agent or by anyone else. `list_secrets`
   returns names.