fix(skills): ignore no-SFX storyboard sentinel

[miguel-heygen]

What

Storyboards can now use sfx: none to explicitly request silence without triggering an SFX lookup or creating a fake none.mp3 metadata entry.

Why

The faceless-explainer workflow treated every non-empty sfx: value as a catalog query. A literal no-SFX sentinel therefore polluted audio_meta.json and could overwrite otherwise-valid audio metadata with a nonexistent asset.

How

The three shared audio adapters now ignore the case-insensitive none sentinel while retaining adjacent real cues. One integration regression test exercises both silent and mixed-cue frames across faceless-explainer, product-launch-video, and pr-to-video.

Test plan

• node --test skills/faceless-explainer/scripts/audio.test.mjs (4/4)
• bunx oxlint on the affected adapters and test
• bunx oxfmt --check on the affected adapters and test
• bun run --cwd packages/cli gen:skills-manifest --check
• git diff --check
• Documentation updated (not applicable)

---

[miguel-heygen]

Closing in the owner-scoped backlog value sweep. This is a narrow lint/audit/docs/example/skills refinement, is currently conflicting, or adds more maintenance surface than its demonstrated user impact justifies. Reopen only if a current reproducible case makes it a priority again.