Add URLs to add, and a migrate command that deletes originals

add now downloads http(s) URLs (with a Chrome user agent) through a
private temporary file, and treats ".", ".." and "/" as "this folder's
contents" so the destination folder is no longer dropped for "..".

migrate is add followed by deleting each original and any directories
left empty. It asks on the terminal before overwriting anything already
in the vault, refusing when there is no terminal, unless --yes is given.

Adding or migrating a directory skips the vault and key file if they
live inside it, so migrate can never ingest or delete them.

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

docs/guide/vault-guide.tex

@@ -522,7 +522,7 @@ for your passphrase. That's the whole daily workflow.
 \section{Putting things in: \texttt{add}}
 
 \begin{shell}
-$ vault add FILE-OR-FOLDER [FOLDER-IN-VAULT]
+$ vault add FILE-OR-FOLDER-OR-URL [FOLDER-IN-VAULT]
 \end{shell}
 
 Give it a file, and it goes in under its own name. Give it a second argument,
@@ -540,6 +540,8 @@ Notice what happened there:
         \file{id/passport-scan.jpg}.
   \item Adding the folder \file{Documents/letters} kept its own name and
         structure: \file{letters/bank.txt}, \file{letters/landlord.txt}.
+  \item Adding \file{.} (or \file{..}) brings in that folder's
+        \emph{contents}, subfolders and all, without a folder name of its own.
 \end{itemize}
 
 \begin{headsup}
@@ -553,9 +555,28 @@ Adding a name that already exists \hl{replaces} it, and vault tells you so.
 The old version is deleted \emph{after} the new one is safely stored, so a
 crash halfway through never leaves you with neither.
 
+\subsection{Straight from the web}
+Give \cmd{add} an \file{http://} or \file{https://} address and vault
+downloads it (as Chrome, so fussy sites play along) and stores it under the
+last part of the address, such as \file{report.pdf}. The download sits in a
+private temporary file only until it's encrypted.
+
+\subsection{Moving in: \texttt{migrate}}
+\cmd{migrate} works exactly like \cmd{add}, but deletes each original once
+it's safely stored, then tidies away any folders left empty. It won't take a
+URL; there's no original to delete.
+
+Because the originals go, \cmd{migrate} checks first: if anything would
+\hl{replace} a file already in the vault, it lists those names and asks
+before touching anything. Answer anything but \texttt{y} and nothing changes.
+With no terminal to ask on, the answer is no. For scripts, \texttt{--yes}
+answers yes up front.
+
 \subsection{What gets skipped}
 When adding a folder, anything that isn't a plain file, such as symbolic
-links, sockets or devices, is skipped with a note. Files larger than 1\,TiB are
+links, sockets or devices, is skipped with a note. So are the vault and key
+themselves, should they live inside the folder, so \cmd{migrate} can never
+eat them. Files larger than 1\,TiB are
 refused. (If you have a single 1\,TiB file, we have questions.)
 
 \section{Seeing what's inside: \texttt{list}}