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>

README.md

@@ -0,0 +1,51 @@
+# go-nc-exapp
+
+Shared Go library for Nextcloud ExApp Services: AppAPI authentication, OCS JSON calls, and per-user ExApp preferences.
+
+Import: `gitea.neitzel.de/konrad/go-nc-exapp`
+
+## v1 scope
+
+**Included**
+
+- **Credentials** — Nextcloud base URL, AppAPI secret, and requesting user identity
+- **AuthHeaders** — outbound AppAPI authorization for OCS and other Nextcloud calls
+- **WithUser** — credentials scoped to a specific requesting user
+- **UserFromRequest** — extract the requesting user from inbound AppAPI-proxied requests
+- **OCSClient** — authenticated OCS calls that always append `format=json`
+- **AppAPIPreferences** — parameterized get/set of a string ExApp preference (caller supplies app id and key)
+
+**Excluded from v1**
+
+- ExApp lifecycle HTTP routes (`/heartbeat`, `/enabled`, …)
+- HaRP listen / `serve()` and unix-socket bootstrap
+- Top-menu, script, and iframe UI registration
+- WebDAV and file storage (see **go-nc-files**)
+- **Visit** folder resolution (see **go-nc-files**)
+
+## Usage
+
+```go
+cred := gonexapp.Credentials{
+    BaseURL:    "https://nextcloud.example",
+    AppID:      "myexapp",
+    AppVersion: "0.1.0",
+    AAVersion:  "1.0.0",
+    AppSecret:  os.Getenv("APP_SECRET"),
+    UserID:     "alice",
+}
+
+prefs := gonexapp.NewAppAPIPreferences(cred, "myexapp", "savedDefault")
+value, err := prefs.Get()
+```
+
+Each ExApp chooses its own preference keys; this library does not hardcode product-specific names.
+
+## Domain language
+
+See [CONTEXT.md](./CONTEXT.md) for AppAPI credentials, requesting user, ExApp preference, and OCS terminology.
+
+## Related
+
+- **go-nc-files** — WebDAV, Working Folder, Saved Default, Visit resolution
+- Workspace ADR 0013 — extraction from CheckDNS