opencode: add the OpenCode sidecar (step 1 — serve lifecycle + port report)

New officer-opencode sidecar (same philosophy as officer-claude): a singleton that owns
an `opencode serve` running from DATA_PATH/opencode-sidecar (created if missing) on a
random port, registers with the API as capability 'opencode', and reports its port via a
new `opencode:server` protocol event. The API stores it (sidecar-server.ts, wired in
server.tsx via getOpenCodeServerUrl). ecosystem.config.cjs runs the sidecar instead of a
bare pm2 serve. Turn-running + API rewiring come in later steps.

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

src/servers/api/chat/opencode/sidecar-server.ts

@@ -0,0 +1,19 @@
+import * as sidecar from '@@/sidecar-registry';
+import { logger } from '../logger';
+
+// The OpenCode sidecar (officer-opencode) starts its `opencode serve` on a random port and reports it
+// here on connect. We remember it so the OpenCode harness always routes to the current server.
+
+let serverPort: number | null = null;
+
+sidecar.on('opencode:server', (msg) => {
+  const port = (msg as { port?: number }).port;
+  if (typeof port !== 'number') return;
+  serverPort = port;
+  logger.info('OpenCode sidecar server registered', { port, url: getOpenCodeServerUrl() });
+});
+
+/** The base URL of the sidecar's OpenCode server, or null if the sidecar hasn't reported in yet. */
+export function getOpenCodeServerUrl(): string | null {
+  return serverPort ? `http://127.0.0.1:${serverPort}` : null;
+}