Add cp, folder mv, get -r, and unlock/lock to remember the key

Filesystem basics: mv now moves folders as well as files, and like mv(1)
a NEW ending in / or naming an existing folder means into it. cp copies a
file, or a folder with -r, by writing new records that share the
original's objects, so it is instant and takes no space; rm already keeps
objects another record still uses. New records go first, so an
interrupted move leaves both names, never neither. get -r extracts a
folder, keeping its structure.

vault unlock [--for 5m] checks the passphrase and starts the binary again
as a hidden __agent process, handing it the suite ID and seed over a pipe.
The agent keeps them in mlocked memory, with core dumps off and (on Linux)
PR_SET_DUMPABLE cleared, serves them on a Unix socket in a 0700 per-user
directory whose owner is checked on every use, and wipes them and exits
at a hard deadline or on vault lock. Commands ask it first unless
VAULT_PASSPHRASE is set, and genkey stops any agent left for that path.

Guide, man pages (new vault-cp, vault-unlock, vault-lock), README and
screenshots updated; the guide's error table is split so it fits.

agent_unix.go

@@ -0,0 +1,39 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build unix
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// undumpable stops other processes of this user reading the agent's memory,
+// where the platform can.
+var undumpable = func() {}
+
+// protectMemory keeps b out of swap and the agent out of core dumps.
+func protectMemory(b []byte) {
+	unix.Mlock(b)
+	unix.Setrlimit(unix.RLIMIT_CORE, &unix.Rlimit{})
+	undumpable()
+}
+
+// privateDir checks dir is a real directory that only this user can use,
+// so nobody else can reach, swap or plant the agent's socket.
+func privateDir(dir string) error {
+	fi, err := os.Lstat(dir)
+	if err != nil {
+		return err
+	}
+	st, ok := fi.Sys().(*syscall.Stat_t)
+	if !fi.IsDir() || !ok || int(st.Uid) != os.Getuid() || fi.Mode().Perm() != 0o700 {
+		return fmt.Errorf("%s is not a private directory of this user", dir)
+	}
+	return nil
+}