feat(infra): add systemd autostart unit for Linux (#8635)

.claude/worktrees/feat-8635/changelog.d/features/8635-systemd-autostart-linux.md

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+- **feat(infra):** add systemd autostart unit for Linux ([#8635](https://github.com/diegosouzapw/OmniRoute/issues/8635))

.claude/worktrees/feat-8635/contrib/systemd/omniroute.service

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+[Unit]
+Description=OmniRoute AI Proxy
+After=network.target network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=$(which omniroute) start
+Restart=on-failure
+RestartSec=5
+Environment=NODE_ENV=production
+
+# Security hardening
+NoNewPrivileges=true
+ProtectSystem=full
+PrivateTmp=true
+
+[Install]
+WantedBy=default.target

.claude/worktrees/feat-8635/tests/unit/systemd-autostart.test.ts

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+import { describe, it } from "node:test";
+import { ok } from "node:assert/strict";
+import { readFileSync, existsSync } from "node:fs";
+
+describe("Systemd autostart (#8635)", () => {
+  const svcPath = "contrib/systemd/omniroute.service";
+  const content = readFileSync(svcPath, "utf-8");
+
+  it("service file exists", () => {
+    ok(existsSync(svcPath));
+    ok(content.length > 200);
+  });
+
+  it("defines required systemd sections", () => {
+    ok(content.includes("[Unit]"));
+    ok(content.includes("[Service]"));
+    ok(content.includes("[Install]"));
+  });
+
+  it("specifies WantedBy=default.target", () => {
+    ok(content.includes("WantedBy=default.target"));
+  });
+});