video-gen: adopt Seedance 2.0 as preferred premium default

Seedance 2.0 is now routed as the top-ranked video generator whenever a
premium gateway is available. Touches the tool layer, scoring engine,
cinematic pipeline, and both skill layers so discovery works from every
entry point.

- tools/video/seedance_video: BETA stability, quality_score=0.95, add
  reference_to_video operation plus 9 img + 3 vid + 3 audio ceilings,
  fix pre-existing upload_image_fal import
- tools/base_tool: surface optional quality_score / success_rate /
  latency fields in get_info so the scorer can read them
- lib/scoring: fix reliability enum-vs-string bug that was pinning every
  available tool to 0.0, switch to overlap coefficient so rich best_for
  descriptions aren't penalized, add premium-cinematic feature bonus
- pipeline_defs/cinematic + cinematic asset-director: add pixabay_music
  and freesound_music, restore pixabay-first music default
- cinematic compose-director: mandatory Remotion preflight at stage entry
- New Layer 3 .agents/skills/seedance-2-0/SKILL.md (8-part prompt
  structure, multi-shot, lip-sync, reference-to-video, provider landscape)
- New Layer 2 skills/creative/prompting/seedance-prompting.md
- Update ai-video-gen, video-gen-prompting, AGENT_GUIDE, INDEX to flag
  Seedance 2.0 as the preferred premium default and make the skill
  discoverable from every routing path

AGENT_GUIDE.md

@@ -604,7 +604,7 @@ The `.agents/skills/` directory is large. When you're not coming in through a to
 | **Composition runtime** | `remotion`, `remotion-best-practices`, `synthetic-screen-recording` (fake terminal/UI demos via Remotion TerminalScene) |
 | **Animation knowledge (generic)** | `gsap-core`, `gsap-timeline`, `gsap-plugins` (SplitText / MorphSVG / DrawSVG / MotionPath / Flip / CustomEase), `gsap-utils`, `gsap-react`, `gsap-performance`, `gsap-scrolltrigger`, `gsap-frameworks`, `framer-motion` (Disney 12 principles), `lottie-bodymovin` (Lottie export) |
 | **Image generation** | `bfl-api`, `flux-best-practices` |
-| **Video generation** | `ai-video-gen`, `ltx2` |
+| **Video generation** | `seedance-2-0` (preferred premium default — cinematic, trailer, multi-shot, synced audio, lip-sync), `ai-video-gen`, `ltx2` |
 | **Audio** | `elevenlabs`, `music`, `sound-effects`, `acestep`, `text-to-speech`, `setup-api-key` |
 | **Avatar / lip-sync** | `avatar-video`, `heygen`, `create-video`, `faceswap`, `video-translate`, `speech-to-text`, `agents` |
 | **Capture** | `playwright-recording` (browser flows), `ffmpeg` (post) |