feat: v0.8.0 edit and save files with conflict detection

- devdesk attach --fs-write SESSION PATH BYTES BASESHA|new: streamed like
  uploads, temp file in the same folder, sha re-check (conflict reply instead
  of overwriting), mode/owner kept, atomic mv; new files with folders, never
  outside the session folder or through a symlink out of it.
- Editor: editable CodeMirror for UTF-8 text, dirty tracking, Ctrl+S,
  Save/Revert, conflict dialog with a disk-vs-yours diff, changed-on-disk
  banner, drafts in localStorage (flushed on pagehide), close/leave guards,
  New file.
- Upload streaming factored into streamIn(), shared by uploads and saves.

Closes #20

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

libexec/devdesk-files.sh

@@ -190,6 +190,81 @@ cmd_git_show() {
     rm -f "$tmp"
 }
 
+# --fs-write SESSION REL BYTES BASE (devdesk#20): save a file from the editor.
+# The content arrives as base64 on stdin after the READY line (like uploads).
+# BASE is the sha256 the editor loaded: if the file changed since, nothing is
+# written and the reply is {conflict: true, sha: <current>}. BASE "new" creates
+# a file (and missing folders) and refuses to overwrite one. Writes go to a
+# temp file in the same folder, keep the mode and owner, and land with mv.
+cmd_fs_write() {
+    [ $# -eq 4 ] || dd_die "usage: devdesk attach --fs-write SESSION PATH BYTES BASESHA|new"
+    local root rel=$2 size=$3 base=$4 target dir name tmp b64len actual current sha mtime
+    root=$(dd_session_root "$1")
+    if ! [[ $size =~ ^[0-9]+$ ]] || [ "$size" -gt "$DEVDESK_FS_MAX" ]; then
+        dd_die "too large: the editor saves files up to $((DEVDESK_FS_MAX / 1024)) KiB"
+    fi
+    [[ $base =~ ^([0-9a-f]{64}|new)$ ]] || dd_die "bad base: $base"
+    case $rel in /* | *$'\n'* | *$'\r'* | '' | */ | . | ..) dd_die "bad path: $rel" ;; esac
+    name=${rel##*/}
+    if [ "$name" = . ] || [ "$name" = .. ]; then dd_die "bad path: $rel"; fi
+    if [ "$base" = new ]; then
+        if [ -e "$root/$rel" ] || [ -L "$root/$rel" ]; then dd_die "already exists: $rel"; fi
+        case $rel in */*) dir=$root/${rel%/*} ;; *) dir=$root ;; esac
+        # Missing folders are created, but never through a symlink out of the
+        # root: check the deepest folder that exists before creating any.
+        local existing=$dir
+        while [ ! -e "$existing" ]; do existing=${existing%/*}; done
+        existing=${existing#"$root"}
+        dd_fs_path "$root" "${existing#/}" >/dev/null
+        mkdir -p -- "$dir" || dd_die "cannot create folder for $rel"
+        dir=${dir#"$root"}
+        dir=$(dd_fs_path "$root" "${dir#/}")
+        target="$dir/$name"
+    else
+        target=$(dd_fs_path "$root" "$rel")
+        [ -f "$target" ] || dd_die "not a file: $rel"
+        dir=${target%/*}
+    fi
+    tmp=$(mktemp --tmpdir="$dir" ".${name}.devdesk-XXXXXX") || dd_die "cannot write in ${dir#"$root"/}"
+    trap 'rm -f "$tmp"' HUP INT TERM
+    # shellcheck disable=SC2017 # integer ceil(size/3)*4: the base64 length, on purpose
+    b64len=$(((size + 2) / 3 * 4))
+    stty raw -echo 2>/dev/null || true
+    printf 'DEVDESK-WRITE-READY\r\n'
+    if [ "$b64len" -gt 0 ]; then
+        head -c "$b64len" | base64 -d >"$tmp" 2>/dev/null || true
+    fi
+    stty sane 2>/dev/null || true
+    actual=$(stat -c %s "$tmp" 2>/dev/null || echo 0)
+    if [ "$actual" != "$size" ]; then
+        rm -f "$tmp"
+        dd_die "save incomplete ($actual of $size bytes)"
+    fi
+    if [ "$base" = new ]; then
+        if [ -e "$target" ]; then
+            rm -f "$tmp"
+            dd_die "already exists: $rel"
+        fi
+        chmod 0644 "$tmp"
+    else
+        current=$(sha256sum -- "$target" | cut -d' ' -f1)
+        if [ "$current" != "$base" ]; then
+            rm -f "$tmp"
+            trap - HUP INT TERM
+            jq -cMn --arg p "$rel" --arg h "$current" '{ok: false, conflict: true, path: $p, sha: $h}'
+            return
+        fi
+        chmod --reference="$target" "$tmp" 2>/dev/null || true
+        chown --reference="$target" "$tmp" 2>/dev/null || true
+    fi
+    mv -f -- "$tmp" "$target" || { rm -f "$tmp"; dd_die "cannot replace $rel"; }
+    trap - HUP INT TERM
+    sha=$(sha256sum -- "$target" | cut -d' ' -f1)
+    mtime=$(stat -c %Y -- "$target")
+    jq -cMn --arg p "${target#"$root"/}" --arg h "$sha" --argjson s "$size" --argjson m "$mtime" \
+        '{ok: true, path: $p, sha: $h, size: $s, mtime: $m}'
+}
+
 # --asset NAME: a lazily loaded part of the web UI (the file viewer's editor),
 # shipped in $DEVDESK_ASSETS and cached by the browser.
 cmd_asset() {