[FEATURE]: reorder and hide session sidebar sections via tui.json #53198

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

The session sidebar shows its sections in a fixed order: Context, MCP, LSP, Todo, Modified Files, followed by sections from TUI plugins according to their slot order. There is no way to change that order or hide a section you never look at. With many MCP servers, for example, the MCP list pushes Todo and Modified Files below the fold, and everybody's priorities differ.

Proposal: two tui.json options.

{
  "sidebar": {
    "order": ["todo", "files", "context"],
    "hidden": ["lsp"]
  }
}

• Built-in sections are named context, mcp, lsp, todo, files. A plugin section uses the id its slot registration got (the plugin id), so plugin-contributed sections work the same way.
• Listed sections come first, in the given order. Sections not listed keep their default order after them, so a section added in a later release, or by a newly installed plugin, never disappears silently.
• The session title is not a section and always stays first.
• Without the option nothing changes.

A follow-up could add mouse drag-and-drop reordering of sidebar section headers, persisted like other runtime TUI settings (kv). I'd keep that in a separate PR because it is more opinionated.

Happy to adjust the shape (e.g. a single list with a -name hide marker) if maintainers prefer.