feat(playlist): orchestrate per-video extraction for playlists

[cmglezpdev]

What this changes

Adds runPlaylistExtraction: identifies the playlist, lists members, loops the unchanged runExtraction per member (nested notes folder, per-video resume/--force), records unavailable members without extracting, aggregates per-member failures, saves the playlist record, and writes the orchestrator prompt.md.

Why

The loop layer that turns the previous slices into an end-to-end pipeline. Infrastructure errors (YTDLP_NOT_FOUND, BOT_CHECK, RATE_LIMITED, PATH_ESCAPE, ROOT_OVERLAP, non-VetaError) abort the run; per-video errors mark that member failed and continue. It returns failedCount instead of throwing so the CLI (next PR) can print the prompt path to stdout before exiting non-zero.

How to verify

pnpm typecheck
pnpm vitest run src/pipeline --exclude '**/.claude/**'

Checklist

• pnpm typecheck and pnpm test pass
• Commits follow Conventional Commits

Scope

• Includes: orchestrator + 7 integration tests (real fake yt-dlp), 4 playlist progress events, PLAYLIST_EMPTY/PLAYLIST_PARTIAL_FAILURE codes (+ mechanical exit-code map entries forced by exhaustiveness).
• Excludes: CLI wiring/stdout contract (next PR), curation flags.

Chain Context

Field	Value
Chain	playlist-support
Tracker PR	#40
Position	5 of 6
Base	feat/playlist-support-03b-playlist-prompt
Depends on	PR 4 (#44)
Follow-up	PR 6 (CLI)
Review budget	569 / 400

Chain Overview

main
 └── #40 tracker: feat/playlist-support
      └── #41 listing → #42 store → #43 packageName → #44 prompt → 04 orchestrator → 05 CLI
          (this PR: 📍 5)