[FEATURE]: markdown preview of project files in the 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

When working in the TUI, markdown files in the project (README, AGENTS.md, plan docs, command specs) are only ever viewable as raw source: no in-app rendered preview. Tables, headings and code blocks require mental parsing, and reading docs means switching editors.

I'd like a /preview command in the session view that:

• opens the fuzzy file picker (the same fs.find-backed search as @-mentions)
• renders the selected file as markdown in a right-hand panel next to the session
• is an inline split on wide terminals and a right-docked overlay on narrow ones (same layout pattern as the sidebar)
• closes by running /preview again; new keybind tokens preview_toggle / preview_close default to unbound so users can map their own

Related: #38047 asks for markdown preview in the web/desktop file viewer. This request is specifically for the terminal TUI, where there is currently no file preview at all.