Adds Overlay.type='files' whose source-of-truth IS the overlay directory
itself. Users can:
* upload arbitrary files / whole folders by dragging from the OS onto a
folder row in the file tree (one POST per file, queue with
concurrency 3, per-file progress in a floating Uploads panel)
* move via drag-and-drop inside the tree (same gesture, source
distinguishes; refuses cycles)
* create / edit / rename / replace through a single editor modal
(text flavor for editable files, binary flavor with replace-upload
for everything else; filename input is the rename surface)
* mkdir empty folders (slashes allowed for nested intermediates)
* stream a folder as a zip download
* delete files and empty folders
Backend is type-agnostic past the new files_routes endpoints, so the
existing mount / spec / overlayfs / expose_server_cfg pipeline is reused
unchanged. is_editable gates the row's edit affordance and the /save
content rules. Three new safe-resolve helpers (write/delete/move) cover
the new operations with the same anchor-and-resolve pattern as listing
and download. FilesBuilder is a no-op so the build subsystem can
dispatch uniformly.
Spec: docs/superpowers/specs/2026-05-09-files-overlay-design.md
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
