add tag-album and clean-playlist-files tasks, script step support, stop fix, rename fix

- New tag-album task: renames tracks to NNN format, sets ID3 tags via mutagen
- New clean-playlist-files task: deletes .cue, .m3u, .nfo and similar junk files
- Pipeline executor now supports script-mode steps (runs directly, no agent)
- Build discography pipeline: convert-audio → clean-playlist-files → prepare → fetch → tag
- Fix stop button: abort signal now kills running agent processes
- Fix job manager: broadcast stopped/error events to WebSocket viewers
- Fix file browser rename: delay focus to avoid context menu close race, left-align input

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/tasks/clean-playlist-files/TASK.md

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+---
+name: Clean Playlist Files
+description: Recursively delete .cue, .m3u, .m3u8, .pls, .wpl, .xspf and other playlist files.
+version: 1
+mode: script
+language: bash
+triggers:
+  - type: directory
+inputs:
+  file_path:
+    type: string
+    description: Path to a directory to clean.
+args: [file_path]
+---
+
+# Clean Playlist Files
+
+Remove playlist and cue sheet files from a directory tree.

seed/tasks/clean-playlist-files/run.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+EXTENSIONS="cue|m3u|m3u8|pls|wpl|xspf|nfo|txt|log|accurip|sfv|md5"
+
+TARGET="${1:-${INPUT_FILE_PATH:-}}"
+
+if [[ -z "$TARGET" ]]; then
+  echo "Error: No directory path provided" >&2
+  exit 1
+fi
+
+if [[ ! -d "$TARGET" ]]; then
+  echo "Error: Not a directory: $TARGET" >&2
+  exit 1
+fi
+
+deleted=0
+
+while IFS= read -r -d '' file; do
+  echo "Deleting: $file"
+  rm "$file"
+  deleted=$((deleted + 1))
+done < <(find "$TARGET" -type f -regextype posix-extended -iregex ".*\.($EXTENSIONS)" -print0 | sort -z)
+
+if [[ $deleted -eq 0 ]]; then
+  echo "No playlist/cue files found in: $TARGET"
+else
+  echo ""
+  echo "Summary: $deleted files deleted"
+fi