feat(deploy): systemd user unit + install script for membershipd

Add deploy/unibus-membershipd.service (Restart=always, binds both planes to
0.0.0.0 for LAN reachability), an idempotent deploy/install.sh that builds the
binary, symlinks the unit, and enables+starts it, plus deploy/README.md with
operate/health instructions.

Restart=always is deliberate: a clean SIGTERM exits 0 and Restart=on-failure
would not restart it, leaving the service silently dead (the sqlite_api gotcha).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/unibus-membershipd.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=unibus membershipd — control plane (rooms, keys, blobs) + embedded NATS/JetStream
+Documentation=https://gitea-dgg044oo04woo4ggcsws4gk0.organic-machine.com/dataforge/unibus
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+WorkingDirectory=%h/fn_registry/projects/message_bus/apps/unibus
+# --bind 0.0.0.0 exposes BOTH the HTTP control plane (:8470) and the embedded
+# NATS data plane (:4250) to the LAN so phones / other PCs can connect.
+ExecStart=%h/fn_registry/projects/message_bus/apps/unibus/membershipd --bind 0.0.0.0
+# Restart=always (NOT on-failure): a clean SIGTERM shutdown exits 0, and
+# on-failure would then NOT restart, leaving the service silently dead. always
+# brings it back regardless of exit code. See .claude/rules/function_tags.md.
+Restart=always
+RestartSec=2
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=default.target