seed default channel(s) on boot

fresh relay seeds configured open channels (RELAY_DEFAULT_CHANNELS,
default general) so a client always has a room. relay-signed 9007
through the normal pipeline; idempotent; never reopens a closed group.

also adds mission/roadmap and separation rationale to the readme.

internal/group/group.go

@@ -267,6 +267,14 @@ func (gs *State) Evaluate(evt nostr.Event) (reject bool, msg string) {
 	return false, ""
 }
 
+// Has reports whether a group with the given id is tracked.
+func (gs *State) Has(id string) bool {
+	gs.mu.RLock()
+	defer gs.mu.RUnlock()
+	_, ok := gs.groups[id]
+	return ok
+}
+
 // IsMember reports whether pk is a member of the group.
 func (gs *State) IsMember(id string, pk nostr.PubKey) bool {
 	gs.mu.RLock()