chat: point OpenCode at a fixed pm2-managed server (fixes empty model picker)

The per-cwd `opencode serve` spawning is replaced by a single fixed server
(http://127.0.0.1:4096, OPENCODE_SERVER_URL) managed by pm2 — added as
`officer-opencode` in ecosystem.config.cjs (cwd = home).

Root-cause fix for the empty model selector: list-models shelled out to
`opencode models`, which failed at runtime on the deployed server (the picker got
only Claude tiers). It now reads the fixed server's GET /config/providers over HTTP —
11ms and reliable — so the curated OpenCode models (Big Pickle, Claude Haiku) show up.

- server-manager.ts — drops spawning; exposes OPENCODE_SERVER_URL + a health check.
- client.ts — createSession no longer binds a directory (sessions live in the one
  server's project).
- send-opencode.ts / opencode-sessions.ts / chat.ts — use the fixed server; drop the
  cwd/home plumbing. Session list/load/delete/rename now hit :4096.

Verified end-to-end against the live server: model list, streaming turn, session
list, and transcript load all work with no spawning.

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

ecosystem.config.cjs

@@ -1,3 +1,5 @@
+const { homedir } = require('os');
+
 module.exports = {
   apps: [
     {
@@ -6,6 +8,15 @@ 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',