[FEATURE]:CLI: session delete command is hard to discover in TUI

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

Problem

There is no obvious way to delete a conversation/session from the TUI.
The docs only mention /new (start a new session), but nothing about
deleting an existing session. Deleting via opencode session delete <id>
requires the user to first run opencode session list, which is not
discoverable in the docs' TUI page.

Suggestion

1. Document the session delete workflow in the TUI docs
   (https://opencode.ai/docs/tui/).
2. Optionally add a delete action in the /sessions list UI
   (ctrl+x l), since the current UI may not expose deletion.