Extract AppAPI auth and OCS preferences from CheckDNS.

Publish gonexapp v1 with Credentials, OCSClient, and parameterized
AppAPIPreferences so ExApps share Nextcloud ExApp plumbing per ADR 0013.

Co-authored-by: Cursor <cursoragent@cursor.com>

CONTEXT.md

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+# go-nc-exapp
+
+Shared Go Library for Nextcloud ExApp Services: AppAPI authentication, OCS calls, and per-user ExApp preferences. ExApps import `gitea.neitzel.de/konrad/go-nc-exapp`. File storage and folder visits live in go-nc-files.
+
+## Language
+
+**AppAPI credentials**:
+The ExApp's shared secret and Nextcloud base URL, plus optional per-request user identity. Used to sign outbound calls to Nextcloud and to read the requesting user from inbound AppAPI-proxied requests.
+_Avoid_: API key (generic), session token
+
+**Requesting user**:
+The Nextcloud user on whose behalf the current ExApp request runs, taken from AppAPI authorization headers. WebDAV and preferences use this user; there is no separate ExApp login.
+_Avoid_: service account (for per-request identity), anonymous
+
+**ExApp preference**:
+A string value stored in Nextcloud for one user and one ExApp, keyed by the ExApp (not admin AppConfig). Libraries expose a parameterized key; each ExApp chooses its own key names.
+_Avoid_: settings file in User Files, instance-wide config
+
+**OCS**:
+Nextcloud's legacy HTTP API surface under `/ocs/v2.php/…`. This Library requests JSON responses (`format=json`) for machine-readable bodies.
+_Avoid_: assuming XML responses, REST-only Nextcloud APIs for ExApp prefs