chat: send cwd on every message instead of reading it back from session metadata

Simpler + more efficient than the previous per-turn GET /session: the client already
has the selected cwd, so it now sends it on every message (it's constant for a session).
The server uses msg.cwd directly for OpenCode's per-turn working-directory system prompt,
and still tags the cwd at creation for listing. Drops the getSession round-trip.

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

src/servers/api/chat/opencode/client.ts

@@ -146,12 +146,6 @@ class ServerConnection {
     return (await res.json()) as OpenCodeSessionInfo[];
   }
 
-  async getSession(sessionId: string): Promise<OpenCodeSessionInfo | null> {
-    const res = await fetch(`${this.baseUrl}/session/${sessionId}`);
-    if (!res.ok) return null;
-    return (await res.json()) as OpenCodeSessionInfo;
-  }
-
   async getMessages(sessionId: string): Promise<OpenCodeStoredMessage[]> {
     const res = await fetch(`${this.baseUrl}/session/${sessionId}/message`);
     if (!res.ok) throw new Error(`opencode GET /session/${sessionId}/message → ${res.status}`);