Add keyboard shortcut for Git panel toggle (⌘6)

[M-Igashi]

Background

Inspired by the Zellij + Yazi + Helix + Lazygit one-key navigation pattern described in 脱IDE。AI時代の開発を支えるRust製ターミナル環境.

The article demonstrates seamless keyboard-driven transitions between file manager → editor → git client. MarkUpsideDown has the same three panels (Sidebar file tree, CodeMirror editor, Git panel) but the Git panel lacks a direct keyboard shortcut.

Current state

Shortcut	Action
⌘1	Toggle Sidebar (file tree)
⌘2	Toggle Editor
⌘3	Toggle Preview
⌘4	Toggle Table of Contents
⌘5	Semantic Search
—	Git panel: no shortcut (click nav button only)

The sidebar has an internal panel system (SidebarPanel = "files" | "git" | "clone") with switchPanel() exported from sidebar.ts, but main.ts does not import or use it.

Proposal

⌘6 — Toggle Git Panel

Behavior:

• If sidebar is collapsed → expand sidebar + switch to git panel
• If sidebar is visible but on a different panel → switch to git panel
• If sidebar is visible and already showing git panel → collapse sidebar

This follows the toggle pattern already used by ⌘1 (sidebar) and ⌘4 (TOC).

Implementation

1. Import switchPanel from sidebar.ts in main.ts
2. Add a toggleGitPanel() function:

   function toggleGitPanel() {
     const isCollapsed = sidebarEl.classList.contains("collapsed");
     const isGitActive = /* check current active panel */;
     if (!isCollapsed && isGitActive) {
       toggleSidebar(); // collapse
     } else {
       if (isCollapsed) toggleSidebar(); // expand
       switchPanel("git");
     }
   }

3. Add ⌘6 handler in the keydown listener (between ⌘5 and the ⌘C handler)
4. Register in command palette:

   { id: "view.gitPanel", label: "Toggle Git Panel", shortcut: "⌘6", category: "View", run: toggleGitPanel }

Files to modify

• ui/src/main.ts — import switchPanel, add keydown handler + command registration
• ui/src/sidebar.ts — may need to export activePanel getter (currently activePanel is a module-level let)

Scope

Small change — no new dependencies, no architectural changes. Keyboard-only workflow improvement.