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/server.tsx

@@ -15,6 +15,7 @@ import { findEntryByProxyId, touchEntry } from './servers/api/dev-server/router'
 import officerWeb from './apps/officer-web/index.html';
 import { startBrowserRelay } from './servers/api/browser/relay';
 import { registerSidecar, unregisterSidecar, handleSidecarMessage } from './servers/sidecar-registry';
+import './servers/api/chat/opencode/sidecar-server'; // subscribe to the opencode sidecar's port report
 import { broadcastEmailNew } from './servers/api/email/email';
 import type { SidecarRegistration } from './servers/sidecar/registration-protocol';
 import { toShellUsername } from './servers/data-path';