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

docker/compose.yml

@@ -108,7 +108,7 @@ services:
       - SENTRY_DSN=${SENTRY_DSN}
       # Flow state survives a restart in Redis; without a host it stays in memory.
       - REDIS_HOST=redis
-      # Everything this installation keeps — the database, the flows, secrets,
+      # Everything this instance keeps — the database, the flows, secrets,
       # artifacts and the user venv — on one volume. The paths below derive
       # from it; they are spelled out because each is load bearing.
       - DATA_DIR=/data
@@ -120,7 +120,7 @@ services:
       # and every issued token with it.
       - MCP_ENABLED=${MCP_ENABLED-false}
       - OAUTH_PRIVATE_KEY_FILE=/data/oauth-key.pem
-      # The portal enrolment, for the same reason: it holds this installation's
+      # The portal enrolment, for the same reason: it holds this instance's
       # credential and the key it pinned, so losing it on a rebuild would mean
       # enrolling again by hand. Absent until someone connects a portal.
       - CLOUD_CONFIG_FILE=/data/cloud.json