Re-shoot the guide's screenshots and add one for migrate

shots.py gains a pseudo-terminal runner so it can answer migrate's
overwrite question, and the guide shows the resulting screenshot.

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

docs/guide/shots.py

@@ -16,6 +16,7 @@ import glob
 import grp
 import html
 import os
+import pty
 import shutil
 import subprocess
 import sys
@@ -94,6 +95,28 @@ class Sandbox:
                            stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
         return self.tidy(p.stdout)
 
+    def run_tty(self, command, passphrase, answer):
+        """Run on a pseudo-terminal, typing answer at vault's [y/N] question."""
+        env = self.env({"VAULT_PASSPHRASE": passphrase})
+        pid, fd = pty.fork()
+        if pid == 0:
+            os.chdir(self.home)
+            os.execve("/bin/bash", ["bash", "-c", command], env)
+        out = b""
+        while b"[y/N]" not in out:
+            out += os.read(fd, 1024)
+        os.write(fd, answer.encode() + b"\n")
+        try:
+            while True:
+                chunk = os.read(fd, 1024)
+                if not chunk:
+                    break
+                out += chunk
+        except OSError:
+            pass
+        os.waitpid(pid, 0)
+        return self.tidy(out.decode().replace("\r\n", "\n"))
+
     def tidy(self, text):
         owner = " %s %s " % (getpass.getuser(), grp.getgrgid(os.getgid()).gr_name)
         return text.replace(self.home, SHOWN_HOME).replace(owner, " sam sam ").rstrip("\n")
@@ -278,6 +301,20 @@ def main():
     t.cmd("find ~/.vault/store/records -type f | wc -l")
     shoot("decoy-hidden", t)
 
+    # migrate: one file is already in the vault, so it asks first
+    box.write("Scans/receipts/march.pdf", os.urandom(48_113))
+    box.write("Scans/receipts/april.pdf", os.urandom(51_870))
+    box.run("vault add Scans/receipts/march.pdf receipts", P)
+    t = Term(box)
+    t.lines.append(prompt() + '<span class="c">vault migrate Scans/receipts</span>')
+    for line in box.run_tty("vault migrate Scans/receipts", P, "y").split("\n"):
+        esc = html.escape(line)
+        if "[y/N]" in line:
+            esc = '<span class="hl">%s</span>' % esc
+        t.lines.append(esc)
+    t.cmd("ls Scans")
+    shoot("migrate", t)
+
     # errors
     t = Term(box)
     t.cmd("vault list", "hunter2", highlight=("incorrect",))

docs/guide/vault-guide.tex

@@ -572,6 +572,8 @@ 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.
 
+\shot{migrate}{Moving a folder in. One file was already there, so vault asked first.}
+
 \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. So are the vault and key