chat: run the /email chat from the email account's storage dir

The email chat's working directory now resolves to
DATA_PATH/<owner>/email_accounts/<accountEmail> (created if missing), so the agent
operates in the selected account's dir (emails.db, attachment_cache, …).

- websocket.ts: new resolveChatCwd — context 'email' → the account dir (via a new
  resolveEmailCwd), 'chat' → the pwd/claude_sessions dir, else the given cwd. Both the
  Claude and OpenCode handlers use it. The account defaults to the owner's first enabled
  account for now; the account selector will pass it as contextId later.
- OpenCode honors the cwd again: send-opencode passes it as the session `directory`
  (client.createSession(directory?)) for context-scoped chats; the general /chat still
  omits it and uses the fixed server's default project. Verified against the live server
  that directory-bound sessions create + list.

No frontend change — the /email panel already sends context:'email'.

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

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

@@ -105,9 +105,13 @@ class ServerConnection {
     return (await res.json()) as T;
   }
 
-  async createSession(title?: string): Promise<string> {
-    // No `directory` — the session lives in the fixed server's own project (its cwd).
-    const session = await this.postJson<{ id?: string }>('/session', title ? { title } : {});
+  async createSession(directory?: string, title?: string): Promise<string> {
+    // `directory` binds the session's working dir (e.g. an email account dir). Omit it for the general
+    // /chat, so the session lives in the fixed server's own project (its cwd).
+    const body: Record<string, unknown> = {};
+    if (directory) body.directory = directory;
+    if (title) body.title = title;
+    const session = await this.postJson<{ id?: string }>('/session', body);
     if (!session.id) throw new Error('opencode POST /session returned no id');
     return session.id;
   }