feat(ext/sw): device management — devices.ts + router handlers

Adds readDevices, addDevice, revokeDevice helpers that read/write
.relicario/devices.json. Router handlers: list_devices, add_device,
revoke_device.

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

extension/src/service-worker/devices.ts

@@ -0,0 +1,55 @@
+/// Device management — reads/writes .relicario/devices.json
+
+import type { GitHost } from './git-host';
+import type { Device } from '../shared/types';
+
+const DEVICES_PATH = '.relicario/devices.json';
+
+interface DevicesFile {
+  devices: Device[];
+}
+
+export async function readDevices(gitHost: GitHost): Promise<Device[]> {
+  try {
+    const raw = await gitHost.readFile(DEVICES_PATH);
+    const text = new TextDecoder().decode(raw);
+    const parsed: DevicesFile = JSON.parse(text);
+    return parsed.devices ?? [];
+  } catch {
+    return [];
+  }
+}
+
+export async function writeDevices(
+  gitHost: GitHost,
+  devices: Device[],
+  message: string,
+): Promise<void> {
+  const content: DevicesFile = { devices };
+  const bytes = new TextEncoder().encode(JSON.stringify(content, null, 2));
+  await gitHost.writeFile(DEVICES_PATH, bytes, message);
+}
+
+export async function addDevice(
+  gitHost: GitHost,
+  device: Device,
+): Promise<void> {
+  const existing = await readDevices(gitHost);
+  if (existing.some((d) => d.name === device.name)) {
+    throw new Error(`device '${device.name}' already exists`);
+  }
+  existing.push(device);
+  await writeDevices(gitHost, existing, `device: add ${device.name}`);
+}
+
+export async function revokeDevice(
+  gitHost: GitHost,
+  name: string,
+): Promise<void> {
+  const existing = await readDevices(gitHost);
+  const filtered = existing.filter((d) => d.name !== name);
+  if (filtered.length === existing.length) {
+    throw new Error(`device '${name}' not found`);
+  }
+  await writeDevices(gitHost, filtered, `device: revoke ${name}`);
+}