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

frontend/tests/panel.spec.ts

@@ -17,7 +17,7 @@ import { api, apiPage, deleteAll } from "./utils/api"
 const flowName = `test_panel_${Date.now().toString(36)}`
 const first = `${flowName}_a`
 const second = `${flowName}_b`
-/** The installation's own panels, put back by the teardown. */
+/** The instance's own panels, put back by the teardown. */
 let panels: PanelDef[] | null = null
 
 test.use({ storageState: "playwright/.auth/user.json" })