tui: open a remote server session as a tab

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Summary

Allow a running OpenCode TUI to open a session from another OpenCode server as a normal in-app tab. Currently, accessing that session requires replacing the current TUI or launching another TUI inside it.

Environment

• opencode version: v0.0.0-beta-18314
• OS: Linux 6.1.21-v8+ aarch64
• Terminal: TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: V2 beta
• Active plugins: None found in config

Reproduction

1. Open the OpenCode TUI and work with multiple in-app session tabs.
2. Identify a session hosted by another OpenCode server.
3. Attempt to open that remote session as another tab in the existing TUI.
4. The available command, opencode2 --server <url> --session <session-id>, starts a separate TUI rather than adding the session to the current TUI.

Expected Behavior

A running TUI should provide a first-class way to open a remote server session as an in-app tab.

The resulting tab could carry its own server and session identity, or OpenCode could provide another remote-session attachment mechanism that avoids nesting or replacing the current TUI.

Actual Behavior

A TUI is connected to one server, and all of its tabs represent sessions from that server. Opening a session from another server requires:

• Exiting and restarting the TUI with --server
• Using a separate native terminal
• Starting another OpenCode TUI inside the current TUI's shell or terminal view

The last option results in a TUI nested inside another TUI.

Additional Context

This is consistently reproducible. The current workaround is to use a separate native terminal window or
replace the current TUI connection.
The requested scope is specifically opening a remote server session as a normal tab in an already-running TUI, not merely restoring the remote server's client-local tab layout.