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>

tests/attach.bats

@@ -620,3 +620,58 @@ blob_body() {
     [ "$status" -eq 0 ]
     [ "$(jq -r '[(.entries | length), .truncated] | @tsv' <<<"$output")" = "$(printf '5\ttrue')" ]
 }
+
+# Save through --fs-write the way the browser does: content as base64 on stdin.
+fs_write() { # SESSION PATH BASE CONTENT-FILE
+    local size
+    size=$(stat -c %s "$4")
+    base64 -w0 "$4" | devdesk attach --fs-write "$1" "$2" "$size" "$3"
+}
+
+@test "fs-write saves atomically, keeps the mode, and refuses stale bases (#20)" {
+    p="$DEVDESK_PROJECTS_ROOT/alpha"
+    printf 'v1\n' >"$p/conf.sh"
+    chmod 0750 "$p/conf.sh"
+    base=$(sha256sum "$p/conf.sh" | cut -d' ' -f1)
+    printf 'v2\n' >"$TMP/new"
+    run fs_write alpha conf.sh "$base" "$TMP/new"
+    [ "$status" -eq 0 ]
+    [ "$(tail -n1 <<<"$output" | jq -r '.ok')" = true ]
+    [ "$(cat "$p/conf.sh")" = v2 ]
+    [ "$(stat -c %a "$p/conf.sh")" = 750 ]
+    [ "$(tail -n1 <<<"$output" | jq -r '.sha')" = "$(sha256sum "$p/conf.sh" | cut -d' ' -f1)" ]
+    # Someone (Claude) changed it since: the old base no longer matches.
+    printf 'v3 from me\n' >"$TMP/mine"
+    run fs_write alpha conf.sh "$base" "$TMP/mine"
+    [ "$(tail -n1 <<<"$output" | jq -r '[.ok, .conflict] | @tsv')" = "$(printf 'false\ttrue')" ]
+    [ "$(cat "$p/conf.sh")" = v2 ]
+    # No temp files left behind
+    [ -z "$(find "$p" -name '*.devdesk-*')" ]
+    # Empty files are fine
+    : >"$TMP/empty"
+    base=$(sha256sum "$p/conf.sh" | cut -d' ' -f1)
+    run fs_write alpha conf.sh "$base" "$TMP/empty"
+    [ "$(tail -n1 <<<"$output" | jq -r '.ok')" = true ]
+    [ ! -s "$p/conf.sh" ]
+}
+
+@test "fs-write creates new files (and folders) but never overwrites or escapes (#20)" {
+    p="$DEVDESK_PROJECTS_ROOT/alpha"
+    printf 'hello\n' >"$TMP/c"
+    run fs_write alpha docs/notes/new.md new "$TMP/c"
+    [ "$(tail -n1 <<<"$output" | jq -r '[.ok, .path] | @tsv')" = "$(printf 'true\tdocs/notes/new.md')" ]
+    [ "$(cat "$p/docs/notes/new.md")" = hello ]
+    run fs_write alpha docs/notes/new.md new "$TMP/c"
+    [ "$status" -ne 0 ]
+    [[ $output == *"already exists"* ]]
+    ln -s "$TMP/outside" "$p/out"
+    for bad in ../escape.txt /etc/x out/pwned.txt out/newdir/x.txt docs/ ""; do
+        run fs_write alpha "$bad" new "$TMP/c"
+        [ "$status" -ne 0 ]
+    done
+    [ ! -e "$TMP/outside/pwned.txt" ] && [ ! -e "$TMP/outside/newdir" ] && [ ! -e "$DEVDESK_PROJECTS_ROOT/escape.txt" ]
+    # A short stream fails without touching anything
+    run bash -c "printf aGVs | '$REPO/bin/devdesk' attach --fs-write alpha docs/notes/new.md 6 $(sha256sum "$p/docs/notes/new.md" | cut -d' ' -f1)"
+    [ "$status" -ne 0 ]
+    [ "$(cat "$p/docs/notes/new.md")" = hello ]
+}