docs: archive event-comms (Concord + Cordn) idea for later evaluation

No decision, no code — parked with the protocol facts, recommended default
(Concord, zero-infra) vs optional Cordn/MLS upgrade, and the open questions.

AGENTS.md

@@ -1892,6 +1892,47 @@ keeps `stateRef` in sync cross-tab; the backup flag is per-account
   (partial — schema + notice done); a few i18n leftovers (some StashPage
   literals) and gray-500/600 contrast.
 
+## Event comms — Concord + Cordn (idea only, NOT started — 2026-10-03)
+
+Parked for later evaluation; **no decision, no code.** The idea: a **host tool**
+for running each event's communications, split by **event visibility** (public
+vs private) — *not* a "staff room". Owner = the event's main host; per event,
+plus a host console that lists/manages them. We likely run **no infrastructure**.
+
+**Recommended default → Concord** (`concordprotocol.org`, MIT; Discord-shaped
+E2EE over Nostr; relays only, no server):
+- CORD-01 private streams · CORD-02 communities (key possession = membership;
+  owner-rooted roster; `community_id`; `community_root`/`control_root`; epochs;
+  Control/Chat/Guestbook planes) · CORD-03 public **and private** channels (each
+  its own key) · CORD-04 roles · CORD-05 invites (revocable links + kind-`3313`
+  direct invites) · CORD-06 rekeys/refoundings (removal = rotate) · CORD-07 A/V ·
+  CORD-08 disappearing messages.
+- ⚠️ Concord "public" is **still E2EE** (open membership, not world-readable) —
+  world-readable chatter is plain Nostr (kind 1/`1111` on the event). Relays must
+  NOT enforce NIP-59's `p` guard (members subscribe by stream author).
+- Kinds: 1059/20013/20014/21059 envelopes; 9/1111/7/5/3302 chat; 3308 control;
+  3303 rekey; 3306/3309/3312 guestbook; 33301 invite bundle; 13303 invite list;
+  33302 community list; 3313 direct invite.
+- ⚠️ **No official JS SDK** (`vector_sdk` is Rust). TS references: **Armada**
+  (Capacitor/TS) and **Accordion** (browser TS). Writing one is a real project
+  (frozen HKDF derivations, the Control-plane **fold**, epochs, byte-identical
+  encoding; interop failures are silent empty rooms).
+
+**Optional upgrade (private events only) → Cordn/MLS**
+(`github.com/Cordn-msg/cordn`; ts-mls coordinator + ContextVM server; per-group
+cursors; KeyPackages; Docker `ghcr.io/cordn-msg/cordn`). Gives **forward secrecy
++ post-compromise security + true member removal**. ⚠️ **Requires a coordinator**
+— if we don't run one, the host supplies one (public default on
+`relay.contextvm.org`, or self-host). Cordn has **no A/V**. We already ship a
+Coordinator settings card + `src/lib/cordn/*` + a crew `coordinator` tag, but the
+app currently uses a Cordn URL **as a relay** and does NOT do coordinator RPC —
+real MLS + RPC is new work.
+
+**Open questions on revisit:** (1) public event = open-join E2EE (Concord) vs
+world-readable (plain Nostr) vs both; (2) one host community with a channel per
+event vs a community per event; (3) is a Concord **private channel** (zero infra)
+enough for private events, or is MLS worth requiring a coordinator?
+
 ## Nostr Protocol Integration
 
 This project comes with custom hooks for querying and publishing events on the Nostr network.