docs: add systemd user service and persistent env setup

Add templates/claude-tokenization-proxy.service (systemd user unit that runs
the proxy at login, restarts on failure) and README sections covering its
installation, lingering for boot-before-login, and permanently setting
ANTHROPIC_BASE_URL across fish, bash/zsh, and environment.d.

templates/claude-tokenization-proxy.service

@@ -0,0 +1,22 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+[Unit]
+Description=claude-code tokenization proxy (ggshield DLP)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+# Edit this to the absolute path where you cloned the repo.
+WorkingDirectory=%h/projects/claude-tokenization-proxy
+# Requires Node >= 23.6 on PATH. If you use nvm/fnm/asdf, replace
+# "/usr/bin/env node" with the absolute path to your node binary
+# (run `which node` to find it).
+ExecStart=/usr/bin/env node src/index.ts
+Restart=on-failure
+RestartSec=5
+# Optional overrides (see README "Configuration"):
+# Environment=PORT=8080
+
+[Install]
+WantedBy=default.target