chat: inject an Officer system prompt telling OpenCode its working directory (step 2)

OpenCode can't set a real per-session cwd (every session runs in the fixed server's
dir), so we tell the model its working directory via a system prompt appended to
OpenCode's own — sent as a system message, so it never appears in the visible chat
(verified against source + live). Claude doesn't need this (it honors cwd natively).

- client.postMessage(…, system?) forwards a `system` string on the message.
- send-opencode builds the Officer prompt from `workingDir` and sends it every turn.
- websocket: workingDir = the resolved cwd, except the general /chat (whose cwd is the
  claude_sessions grouping placeholder) uses the user's home.

Verified live: with the prompt, the model reports the injected dir as its cwd.

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

src/servers/channels/send-opencode.ts

@@ -14,7 +14,8 @@ type OpenCodeStreamingParams = {
   username: string;
   prompt: string;
   sessionKey: string;
-  cwd?: string;
+  cwd?: string; // logical cwd for session tagging/listing (metadata.officer.cwd)
+  workingDir?: string; // the dir the model should treat as its cwd (Officer system prompt)
   model?: string;
   role?: string;
   resumeSessionId?: string;
@@ -32,6 +33,18 @@ function splitModel(model: string): { providerID: string; modelID: string } {
   return { providerID: model.slice(0, slash), modelID: model.slice(slash + 1) };
 }
 
+// OpenCode can't set a real per-session cwd (every session runs in the fixed server's dir), so we tell
+// the model its working directory via an appended, non-visible system prompt. Sent on every turn.
+function officerSystemPrompt(cwd: string): string {
+  return [
+    `Session working directory: ${cwd}`,
+    '',
+    `For this session, treat \`${cwd}\` as your current working directory. Resolve relative paths, globs, and file references against it, and run shell commands from inside it (\`cd\` into it, or use absolute paths beneath it). When the user says "here", "this directory", or gives a relative path, they mean a location within \`${cwd}\`.`,
+    '',
+    `Unless the user explicitly directs you elsewhere, keep your work focused on \`${cwd}\`, its files, and its subdirectories.`,
+  ].join('\n');
+}
+
 export async function sendOpenCodeStreaming(params: OpenCodeStreamingParams): Promise<OpenCodeStreamingHandle> {
   logger.info('OpenCode streaming exec', { sessionKey: params.sessionKey, model: params.model });
 
@@ -60,9 +73,10 @@ export async function sendOpenCodeStreaming(params: OpenCodeStreamingParams): Pr
   unsub = conn.subscribe(sessionId, mapper);
 
   const { providerID, modelID } = splitModel(params.model ?? '');
+  const system = params.workingDir ? officerSystemPrompt(params.workingDir) : undefined;
 
   // Fire the turn; assistant tokens + tool calls stream back over the SSE subscription above.
-  conn.postMessage(sessionId, providerID, modelID, params.prompt).catch((err) => {
+  conn.postMessage(sessionId, providerID, modelID, params.prompt, system).catch((err) => {
     logger.error('OpenCode postMessage failed', { sessionKey: params.sessionKey, error: String(err) });
     params.onEvent({ type: 'error', message: 'Failed to send message to OpenCode' });
     unsub();