feat(irc): irc.profullstack.com host, OS-user (tilde.town) gate, premium channels

Hostname: serve the network as irc.profullstack.com (new IRC_DOMAIN var,
default irc.<root-of-DOMAIN>). Caddy serves an irc.profullstack.com site so it
gets a Let's Encrypt cert; ergo-refresh-certs copies that into Ergo for 6697.
Needs an A record irc.profullstack.com -> the box (self-signed until it resolves).

Members are OS users (tilde.town model): setup.sh reconciles a real OS account
per member dir (root-side, on each deploy + the 15-min timer; nologin shell, so
identity-only — no shell access). The IRC auth-script now gates on
`getent passwd` with uid>=1000 instead of the member dir, so "OS user" == member.

Premium channels: free members may /join; creating channels is a premium perk.
The ssh irc@ client gains /create #name (premium-gated via ensurePremium): it
joins the fresh channel and registers it with ChanServ as the member's founder.
v1 gate is route-level (operator-only-creation left off); external-client
creation hardening is a follow-up. See docs/irc.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/ergo/auth-script.sh

@@ -4,21 +4,22 @@
 # membership. setup.sh installs this to /usr/local/bin/ergo-auth-member and
 # wires it into /etc/ergo/ircd.yaml (accounts.auth-script).
 #
-# "Member" == a user with a home dir under the AgentBBS users dir (created when
-# someone registers via `ssh join@`). IRC is members-only, so a login is
-# approved iff the requested account name maps to such a dir. The passphrase is
-# intentionally IGNORED — membership (a filesystem dir) IS the credential, by
-# design (see docs/irc.md). Anyone who knows a member's name can connect as
-# them; that tradeoff was chosen deliberately for this private, TLS-only network.
+# AgentBBS members are real OS users (tilde.town model; setup.sh provisions an
+# OS account per member). IRC is members-only, so a login is approved iff the
+# requested account name is a real OS user with uid >= MIN_UID (which excludes
+# system accounts like root/ergo/agentbbs). The passphrase is intentionally
+# IGNORED — membership (being an OS user) IS the credential, by design (see
+# docs/irc.md). Anyone who knows a member's name can connect as them; that
+# tradeoff was chosen deliberately for this private, TLS-only network.
 #
 # Protocol (Ergo): one JSON object on stdin per attempt, one JSON line on stdout
 # then exit. Input keys: accountName, passphrase, certfp, ip. Output:
 # {"success":bool,"accountName":str,"error":str}.
 #
-#   args: ["<users-dir>"]   # defaults to /var/lib/agentbbs/users
+#   args: ["<min-uid>"]   # defaults to 1000
 set -uo pipefail
 
-USERS_DIR="${1:-/var/lib/agentbbs/users}"
+MIN_UID="${1:-1000}"
 
 # Always emit valid JSON and exit 0 — Ergo reads the JSON, not the exit code;
 # a non-zero exit / no output is treated as a script error, not a clean deny.
@@ -35,14 +36,22 @@ acct="$(printf '%s' "$line" | jq -r '.accountName // ""' 2>/dev/null || true)"
 # certfp-only attempts carry no account name; we don't support cert auth here.
 [ -n "$acct" ] || deny "membership requires an account name"
 
-# Defense in depth against path traversal. IRC account names are a restricted
-# charset anyway, but never let one escape USERS_DIR.
+# Restrict to plain login names (defense in depth; IRC names are limited anyway).
 case "$acct" in
-  *[!A-Za-z0-9._-]* | "." | ".." | *..* | */* ) deny "invalid account name" ;;
+  *[!A-Za-z0-9._-]* | "." | ".." ) deny "invalid account name" ;;
 esac
 
-if [ -d "$USERS_DIR/$acct" ]; then
+# Resolve the OS account; getent passwd returns name:passwd:uid:gid:...
+entry="$(getent passwd "$acct" 2>/dev/null || true)"
+[ -n "$entry" ] || deny "not a member"
+
+uid="$(printf '%s' "$entry" | cut -d: -f3)"
+case "$uid" in
+  ''|*[!0-9]*) deny "not a member" ;;
+esac
+
+if [ "$uid" -ge "$MIN_UID" ]; then
   printf '{"success":true,"accountName":"%s"}\n' "$acct"
 else
-  deny "not a member"
+  deny "system accounts cannot use IRC"
 fi