Ship the Files tab container-side only

Four successive audits found the same thing: host filesystem paths crossing
IPC is where the criticals in this work live. The most recent one found the
`link(2)` upload reservation returning success against a *directory* (linking
into it, leaving permanent stray files, and via a symlink-to-directory writing
outside the validated write root), failing every upload permanently on any
filesystem without hard links, and the post-resolution credential check
weakened from a general rule to an eleven-name denylist.

Rather than fix that a fifth time, the Files tab ships as what it is good at:
a browser, viewer and renamer that never touches the host.

Removed: `upload_file_to_container`, `download_container_file`, and everything
that existed only for them — the whole reservation (`UPLOAD_RESERVATION_SCRIPT`,
`reserve_upload_destination`, the placeholder rollback, `exec_oneshot_as_within`
which had no other caller), `stream_container_file_to_host`, `ChannelReader`,
`save_to_host`, the download ceiling, and the collision marker with its
frontend contract. On the frontend: the upload button, the pane's
`onDragDropEvent` handler, both "Save to host…" affordances, `uploadPaths` /
`downloadFile` / the overwrite prompt, and `OverwriteConfirmModal`.
`lib/uploadErrors.ts` is now `lib/refusalText.ts` and keeps only the half that
turns any backend refusal into the sentence a person reads.

Kept, and not weakened: `upload_host_file_to_terminal` and
`download_container_backup`. They predate this work, their hardening is a real
improvement over main, and they are now the whole answer to "how do I get a
file in or out" — drop it on the Terminal, or Back up container. The drop gate
(`lib/dropTarget.ts`, `PaneVisibility`) is untouched.

`resolve_host_path` gets the general hidden-component rule back. Round 3
replaced it with `HOST_CREDENTIAL_DIRS`, which is allow-by-omission for the
rest of `$HOME`: `~/.local/bin` (write there and you own the user's next shell
command), `~/.password-store`, browser profiles and `~/.pki/nssdb` were all
reachable through a planted symlink with a visible name — verified against a
real home directory, and all five refused now. It over-catches `.pnpm` and
`~/.cache`; for two occasional callers that is the cheaper mistake, and the
refusal says which folder it resolved through.

Two defects fixed while in here:

  * A symlinked directory listed as empty. `find` defaults to `-P`, which does
    not follow a symlink even as the starting point, so `-mindepth 1` discarded
    the only match and a real directory rendered as "Empty directory" — a
    first-order defect now that browsing *is* the feature. `-H` follows the
    starting point and nothing else, so a loop is `ELOOP` rather than a walk
    that does not end; verified against a live container for a symlinked
    directory, a broken link and a loop. `find`'s errno for the loop case is
    now a sentence.
  * `finish_download`'s replace path fired on *any* rename failure with a
    destination present — a vanished partial, a permission error, a directory
    at the destination — and deleted the user's file to complete a move that
    could not complete. It is now fenced to Windows (where a rename onto an
    existing path genuinely fails) and to a partial that still exists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

HOW-TO-USE.md

@@ -228,7 +228,7 @@ buttons. Below that are six tabs:
 | **Sessions** | Past Claude Code conversations stored on this project's config volume, each with a **Resume** button |
 | **Automation** | The scheduled tasks running inside this container — see [Automation & Scheduled Tasks](#automation--scheduled-tasks) |
 | **Config** | All per-project configuration — see [Project Configuration](#project-configuration) |
-| **Files** | Browse, download and upload files inside the container |
+| **Files** | Browse, view and rename files inside the container — see [Files](#files) for how files get in and out |
 | **Browser** | Watch — and take over — the browser Claude is driving with Playwright, see [The Browser Tab](#the-browser-tab) |
 
 ### Sessions
@@ -351,7 +351,7 @@ it. The sidebar row carries only the two hover controls.
 | **Force stop** | Project Home header | Starting / Stopping | Interrupts a transition that is stuck |
 | **Open Claude Terminal** | Project Home header; sidebar hover control; `Ctrl+T` | Running | Opens a new Claude Code terminal tab |
 | **Shell** | Project Home header | Running | Opens a bash login shell tab in the container (no Claude Code) |
-| **Files** | Project Home header, and the **Files** tab | Running | Switches to the Files tab to browse, download and upload files |
+| **Files** | Project Home header, and the **Files** tab | Running | Switches to the Files tab to browse, view and rename files inside the container |
 | **Config** | The **Config** tab | Always | Per-project configuration (most fields need the container stopped) |
 | **Back up container** | **⋯** overflow menu | A container exists | Saves a `.tar.gz` archive of the container to a location you choose |
 | **Reset container…** | **⋯** overflow menu | Stopped or Error | Destroys the container, snapshot image and both volumes, then recreates from the base image (wipes `~/.claude`) — asks first |
@@ -1161,16 +1161,43 @@ When you scroll up in the terminal to review previous output, a **Jump to Curren
 
 ### Files
 
-The **Files** tab of Project Home browses inside a running container. You can:
+The **Files** tab of Project Home browses inside a running container. It works entirely on the
+container side — it never reads or writes anything on your own machine. You can:
 
-- **Browse** the container filesystem, starting at `/workspace`, with breadcrumb navigation
-- **Save to host…** — copy any file out to a location you pick. This is the way to get a file out
-  of a container; there is one button per file entry, and the file viewer offers it too
-- **Upload file** from your host into the current container directory — or **drop files straight
-  onto the pane** from your desktop, which uploads them into the directory on screen
+- **Browse** the container filesystem, starting at `/workspace`, with breadcrumb navigation.
+  Double-click a folder to open it, or the `..` row to go up; the arrow keys, Home and End move
+  between rows and Enter opens the selected one
+- **View** a file — double-click it, or press Enter. Text files and images render in a read-only
+  viewer
+- **Rename** an entry, from the row's Rename button or by pressing `F2`. A rename never moves a
+  file between folders
+- **New folder** in the directory on screen
 - **Refresh** the directory listing at any time
 
-The listing shows file names, sizes, and modification dates.
+The listing shows file names, sizes, and modification dates, and marks symbolic links.
+
+#### Getting files in and out
+
+The Files tab deliberately does **not** copy files between your computer and the container. There
+are two supported routes, and they are the ones to use:
+
+- **To get a file in:** drag it from your desktop and **drop it onto the Terminal tab**. The file
+  is copied into the container and its path is typed into the terminal for you, ready to hand to
+  Claude Code. (You can also drop it onto the terminal's *Following* toggle — the whole pane is a
+  drop target.)
+- **To get files out:** use **Back up container** in Project Home's **⋯** overflow menu. It writes
+  a `.tar.gz` of the workspace and the container's `~/.claude` config to a location you choose.
+  For a single file, `cat` it in a terminal, or work in a project folder that is mounted from your
+  host in the first place — those files are already on both sides.
+
+Both routes refuse a destination whose path passes through a hidden folder — anything with a
+component beginning with `.`, such as `~/.ssh`, `~/.local/bin` or `~/.config`. That rule catches
+more than it strictly needs to (a path that happens to resolve through `~/.cache` or
+`node_modules/.pnpm` is refused as well), and the refusal says which folder tripped it. Choose a
+visible location such as `~/Documents` or `~/Downloads`.
+
+If you already keep the project in a folder mounted into the container, the simplest answer is
+usually neither of the above: edit the file on your host and it is already inside.
 
 ### Terminal Rendering