Add folder workspaces: hideable sidebar + native tabs + macdown .

[fluxa]

Summary

Adds folder workspaces to MacDown 3000: browse a folder of Markdown files in a hideable sidebar and open them as native macOS window tabs, instead of only opening individual files. Reachable from a new File → Open Folder… command and from the command line with macdown ..

This is an additive feature — the editor/preview pipeline and existing single-file behaviour are untouched, and there are no new dependencies.

What you get

• Folder sidebar (an NSOutlineView source list) showing subfolders and Markdown files (.md / .markdown), dotfiles excluded.
• Open files as native tabs — double-click (or Enter) opens a file in the same window as an OS tab; each file is its own document with its own undo/dirty state. Re-opening an already-open file just focuses its tab.
• macdown . — the CLI learns to accept a directory and opens it as a workspace (files still open as before; macdown file.md is unchanged).
• Hide/show with ⌘\, Reveal in Finder / Copy Path context menu, and live refresh when the folder changes on disk.
• Seamless tab switching — sidebar width, visibility, and expanded folders stay in sync across a workspace's tabs (editor/preview split and per-file selection stay per-tab).

How it works

MacDown is a standard NSDocument app (one document per window). Rather than refactor that, this leans into native window tabbing (addTabbedWindow:): each file stays its own MPDocument, and the OS merges their windows into one tabbed window. The sidebar is added per-window by nesting the existing editor/preview split inside an outer NSSplitView.

New components (all under MacDown/Code/Sidebar/):

• MPFileNode — filesystem-tree model (filtering + folders-first sort).
• MPFolderWatcher — recursive FSEvents watcher (respects the existing remote-volume guard).
• MPFolderSidebarViewController — the source list + open/activation + context menu.
• MPSidebarSyncCoordinator — shares sidebar state across tabs.

Wiring:

• MPDocument gains workspaceRootURL, sidebar install/toggle, and the open-as-tab flow.
• MPMainController gains Open Folder… and inserts its menu items by member selector (so they install in non-English locales too).
• macdown-cmd routes directory arguments to a new shared-defaults key.

Testing

• New XCTest coverage for the model, watcher, sync coordinator, directory routing, menu installation, tab-reuse lookup, and workspace-root propagation. The full suite passes (the pre-existing HGMarkdownHighlighterTests theme failures are unrelated to this change).
• Window tabbing and the view nesting can't be exercised headlessly; those were verified manually in the running app: Open Folder… / macdown ., double-click-to-tab + tab reuse, ⌘\ toggle, live refresh, Reveal/Copy Path, and cross-tab sync.

Notes / design decisions

• Per-tab sidebar, kept in sync. Native tabs are separate windows, so each carries its own sidebar; the coordinator keeps width/visibility/expansion consistent (no-op updates are ignored to avoid feedback loops, and width only syncs on a real divider drag).
• No persistence across launches (start fresh), other than the sidebar's default width.
• Happy to split this into smaller commits, adjust scope, or iterate on anything.

Closes/relates to: (feel free to link a folder-browsing feature request if one exists)

[schuyler]

@fluxa pretty exciting, thank you for submitting. this is a feature that several folks have asked for. I'll have a review done and send notes back. we can try to target the 3000.0.8 release for this feature.

[fluxa]

Thanks, sounds great, ready for your notes whenever.