docs: initial SelfPhoneNG spec

Fix leftover __OP_* dialplan variables from the openphone rename (all
readers use SP_*). Record the reverse-proxy decision in 00 §10.

README.md

@@ -0,0 +1,14 @@
+# SelfPhoneNG
+
+A fully open source, self-hosted business phone system: extensions, desk phones and a
+browser softphone, voicemail to email, IVRs, ring groups, queues, office hours, call
+recording, live dashboard, SMS and phone provisioning. Bring your own SIP trunk and
+numbers (reference provider: voip.ms). Built on Asterisk 24 LTS + TypeScript + PostgreSQL 18.
+
+**Status:** specification stage. Nothing is built yet.
+
+- Start here if you are writing code (human or AI): [`AGENTS.md`](AGENTS.md)
+- The full spec: [`docs/spec/`](docs/spec/00-overview.md)
+- Build order: [`docs/spec/09-roadmap.md`](docs/spec/09-roadmap.md)
+
+License: AGPL-3.0-or-later.