feat: files pane like VS Code: tree, tabs, go to file

- Explorer tree with inline expand/collapse and keyboard navigation, git
  colours, change dots, dimmed gitignored entries (fs-list now flags them),
  file-type badges, resizable explorer.
- Editor tabs: preview tab semantics, instant switching with scroll kept,
  in-place reload when Claude changes an open file.
- Go to file: devdesk attach --fs-find (git ls-files or capped find) and a
  fuzzy-ranked palette (CommandPalette gained custom search/async lists).
- Per-session memory of folders and tabs; empty state with recents.

Closes #37

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -46,7 +46,7 @@ The tmux server runs in its own unit, so every Claude process lives in **that**
   - **Ask Claude** types an investigation prompt into the current session without sending it.
   - Configure `DEVDESK_GRAFANA_URL`, plus a Viewer token in `DEVDESK_GRAFANA_TOKEN_FILE`.
 - **Repo / CI panel:** branch, ahead/behind, uncommitted changes, HEAD's Gitea CI status and open PRs for the session's folder. When CI fails, **Ask Claude to fix CI** types a prompt to read the failed job's log.
-- **Files** (Ctrl+Shift+E): browse the session's folder beside the terminal, read files with syntax highlighting, preview markdown (sanitized, with its relative images) and images, and review uncommitted changes: git status with line counts, a **C** badge on files Claude edited, and a diff against HEAD. **→ Claude** types `@path` into the prompt. Read-only for now. Paths can't leave the session folder. The editor bundle (`/usr/share/devdesk/assets/editor.js`) loads over ttyd the first time and is cached by the browser.
+- **Files** (Ctrl+Shift+E), laid out like VS Code: an explorer tree (git colours, gitignored entries dimmed) and an editor with tabs (a click opens a preview tab, a double-click keeps it), **Go to file** (Ctrl+P in the pane), markdown previews (sanitized, with relative images), images, and a Changes view of uncommitted files with line counts, a **C** badge on files Claude edited, and diffs against HEAD. **@Claude** types `@path` into the prompt. Tabs and open folders are remembered per session. Read-only for now. Paths can't leave the session folder. The editor bundle (`/usr/share/devdesk/assets/editor.js`) loads over ttyd the first time and is cached by the browser.
 - **Context:** a Claude `statusLine` command (`devdesk-statusline`) snapshots each session's model, context-window use and cost to `/var/lib/devdesk/context/<session>.json`. The Sessions panel shows it as a bar (cost in the tooltip), and Claude's own footer shows a one-line summary.
 - **Light mode:** the terminal switches palettes with the page. DevDesk starts Claude with its `dark-ansi` theme so Claude draws with the terminal's 16 colours, and the light palette maps bright white to ink; tmux and the status line use ANSI colours too. A session picks up Claude's theme the next time Claude starts in it (`/exit`, then ⏎ resumes the same conversation).
 - **Phone:** a key bar replaces the shortcut hints (Esc, Tab, ⇧Tab, ^C, ↑, ↓, ⏎, Claude/Shell window), and the panels open as a drawer.