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>

ecosystem.config.cjs

@@ -1,5 +1,3 @@
-const { homedir } = require('os');
-
 module.exports = {
   apps: [
     {
@@ -8,21 +6,18 @@ module.exports = {
       args: 'start',
       watch: false,
     },
-    {
-      // The fixed OpenCode server the chat harness talks to (http://127.0.0.1:4096).
-      name: 'officer-opencode',
-      script: homedir() + '/.opencode/bin/opencode',
-      args: 'serve --port 4096 --hostname 127.0.0.1',
-      cwd: homedir(),
-      env: { HOME: homedir() },
-      watch: false,
-    },
     {
       name: 'officer-claude',
       script: 'bun',
       args: 'run src/servers/sidecar/claude/index.ts',
       watch: false,
     },
+    {
+      name: 'officer-opencode',
+      script: 'bun',
+      args: 'run src/servers/sidecar/opencode/index.ts',
+      watch: false,
+    },
     {
       name: 'officer-email',
       script: 'bun',