feat: open single file from disk with linked autosave

[ijbo]

What

Adds an "Open File" button to the workspace sidebar that opens a single file from disk via showOpenFilePicker and keeps it linked — edits autosave straight back to that exact file on disk, independently of the existing folder-backed workspace mode.

Originally requested because clicking New Document → Open Folder only let you connect a whole folder; there was no way to open and edit one file from disk.

How it works

• disk-workspace.js — new single-file handle API: openSingleFile, linkSingleFile, hasSingleFile, writeSingleFile, unlinkSingleFile, unlinkAllSingleFiles, restoreSingleFiles. Handles are persisted to IndexedDB (file:<id> keys) so links survive reloads.
• workspace.js — M.wsOpenDiskFile() reads the picked file into a deduped, manually-named workspace entry and links its handle; delete/rename paths are guarded so single-linked files never trigger folder-mode I/O.
• cloud-share.js — the real per-keystroke autosave now writes back to the linked file.
• index.html — the ws-open-file button.

Bugs found via multi-agent review and fixed

A deep multi-agent review (4 dimensions × adversarial verification, 16 verified findings) caught that the first implementation looked done but didn't actually save to disk on the main path. All fixed:

Severity	Bug	Fix
🔴 critical	Edits never reached disk — keystroke autosave (saveToLocalStorage) only had a folder-mode branch	Single-file write-back branch + pagehide/beforeunload flush
🟠 major	Picked handle was read-only	Request readwrite permission in the click gesture on open
🟠 major	No write serialization → out-of-order corruption	Per-id write queue
🟠 major	Delete/rename could destroy a same-named file in a connected folder	hasSingleFile guards skip folder-mode removeEntry/renameFileInPath
🟠 major	Deleting the last file truncated the linked disk file	Unlink before clearing
🟠 major	Folder connect/reconnect orphaned single-file handles	unlinkAllSingleFiles() on rebuild
🟡 minor	Redundant write on open, orphan-handle leak, silent persist failure, .md forced onto .txt, unwanted cloud copy	Each addressed

Testing

• Verified live in the browser preview: keystroke autosave writes to disk; serialization preserves order under slow I/O; zero redundant writes on open; permission requested on open; delete/rename guards behave.
• disk-workspace.spec.js: 30 passing, no regressions (the one pre-existing modal-click flake fails identically on main).
• ESLint: no new errors/warnings on changed files.
• Security check (pre-commit): 0 critical.

Note: Chromium-only (File System Access API), same as the existing "Open Folder". The button auto-hides where unsupported.

🤖 Generated with Claude Code