docs: explain project directories and sessions in the TUI guide

Feature hasn't been suggested before.

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Describe the enhancement you want to request

A new OpenCode user, especially one coming from a traditional desktop app or IDE, may not immediately understand how OpenCode organizes projects, workspaces, directories, and sessions.

I suggest adding a short section after Initialize and before Usage that explains this mental model and the common workflows around it, such as:

• what project/directory OpenCode is currently working in;
• what a workspace represents and how it relates to the project;
• how sessions relate to the current workspace/project;
• starting a separate task with /new;
• returning to an existing session with /sessions;
• starting OpenCode for another project with opencode /path/to/project.

This would give newcomers a good conceptual model before the guide moves into individual ways of using OpenCode.

The issue does not name a file; start with the TUI guide section around Initialize and Usage. Add a short section explaining projects, directories, workspaces, and sessions, including /new, /sessions, and opencode /path/to/project, then confirm the guide gives newcomers a clear mental model before the usage details.