feat(md): launch MarkText detached, with an optional read-only sandbox

`md` forwards every argument to marktext untouched except two flags of its
own: `--read-only`/`-r` and `--foreground`. By default it detaches via
`bkg`, so the shell stays usable and the editor outlives the window that
launched it.

MarkText has no read-only mode, so `-r` sandboxes it with firejail, binding
each named file read-only. The subtlety is that MarkText is single-instance:
a plain launch hands the file to an already-running, unsandboxed, writable
window and exits, silently defeating the sandbox. `-r` therefore also passes
a private `--user-data-dir`, which forces an independent instance the
read-only bind actually covers, plus `--no-sandbox`, since Electron's own
sandbox needs the user namespaces firejail has already taken away.

Flags whose entire purpose is terminal output (`--version`, `-v`/`--verbose`,
`--debug`) imply `--foreground`; backgrounding them would send the output
you asked for to /dev/null.

The function is autoloaded and so never shadows an `md` function or alias
defined elsewhere -- fish only looks in functions/ when nothing named `md`
exists. A real `md` *binary* would be shadowed, so the body hands off to it
verbatim whenever marktext is not installed. No conf.d file and no
opinionated guard: `md` is a novel name rather than a command shadow, the
same as `bkg` and `detach`.

functions/md.fish

@@ -0,0 +1,124 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# CATEGORY
+#   03-editors-and-viewers
+#
+# DEPENDENCIES
+#   marktext, firejail, bkg
+#
+# SYNOPSIS
+#   md [-r] [--foreground] [marktext-args...] [FILE...]
+#
+# DESCRIPTION
+#   Opens files in MarkText, detached from the terminal so the shell stays
+#   usable and the editor survives closing the window that launched it.
+#
+#   Every argument is forwarded to marktext untouched except the two flags
+#   below, which md consumes itself. marktext's own flags (--new-window,
+#   --safe, --disable-gpu, ...) therefore work exactly as documented in
+#   marktext --help.
+#
+#   Flags whose entire purpose is terminal output -- --version, -v/--verbose
+#   and --debug -- imply --foreground, since backgrounding them would send
+#   the output you asked for to /dev/null.
+#
+#   --read-only sandboxes the editor with firejail so saving fails instead of
+#   overwriting the file. MarkText has no read-only mode of its own.
+#
+# ARGUMENTS
+#   FILE...            Markdown files to open
+#   -r, --read-only    Open sandboxed, with every named file bound read-only
+#   --foreground       Run in the foreground; do not detach
+#   -h, --help         Show this help message
+#
+# EXIT STATUS
+#   0  MarkText launched (or, with --foreground, exited successfully)
+#   1  --read-only was requested without firejail or without an existing file
+#
+# EXAMPLE
+#   md README.md
+#   md --read-only NOTES.md
+#   md --foreground --debug draft.md
+#   md --new-window one.md two.md
+#
+# NOTES
+#   This file is autoloaded, so it never shadows an md function or alias
+#   defined elsewhere -- fish only looks here when nothing named md exists.
+#   A real md *binary* would be shadowed, so md hands off to it verbatim
+#   whenever marktext is not installed.
+function md --wraps marktext --description 'Launch MarkText detached from the terminal'
+    # Without marktext this wrapper has nothing to offer, so give the name
+    # back to whatever md the system does provide.
+    if not type -q marktext
+        command md $argv
+        return $status
+    end
+
+    __fish_help_header (status current-function) $argv; and return 0
+
+    # Split our own two flags out of the argument list. Everything else is
+    # marktext's business and is forwarded verbatim.
+    set -l read_only 0
+    set -l foreground 0
+    set -l args
+    for arg in $argv
+        switch $arg
+            case -r --read-only
+                set read_only 1
+            case --foreground
+                set foreground 1
+            case '*'
+                set -a args $arg
+        end
+    end
+
+    # Backgrounding a flag that exists to print something defeats it.
+    for flag in --version -v --verbose --debug
+        if contains -- $flag $args
+            set foreground 1
+            break
+        end
+    end
+
+    set -l cmd marktext $args
+
+    if test $read_only -eq 1
+        if not type -q firejail
+            echo "md: --read-only requires firejail" >&2
+            return 1
+        end
+
+        # firejail rejects relative --read-only targets.
+        set -l ro_flags
+        for arg in $args
+            if test -e $arg
+                set -a ro_flags --read-only=(path resolve $arg)
+            end
+        end
+        if test (count $ro_flags) -eq 0
+            echo "md: --read-only needs an existing file to protect" >&2
+            return 1
+        end
+
+        # MarkText is single-instance: a plain launch hands the file to an
+        # already-running -- unsandboxed, writable -- window and exits,
+        # silently defeating the sandbox. A private user-data directory
+        # forces an independent instance that the read-only bind covers.
+        set -l cache (set --query XDG_CACHE_HOME; and echo $XDG_CACHE_HOME; or echo "$HOME/.cache")
+        set -l ro_data "$cache/marktext-readonly"
+        mkdir -p $ro_data
+        or return 1
+
+        # --no-sandbox: Electron's own sandbox needs user namespaces that
+        # firejail has already taken away.
+        set cmd firejail $ro_flags marktext --no-sandbox --user-data-dir=$ro_data $args
+    end
+
+    if test $foreground -eq 1
+        $cmd
+        return $status
+    end
+
+    bkg $cmd
+end