Add scaffold command for new videos - #4

Fixes #3

Adds npm run new -- <number> <slug> that scaffolds the two files every new video needs:

• posts/<number>-<slug>.json — slug/title/tagline/script placeholders and an empty claims[]
• src/scenes/data/<number>.ts — a minimal valid SceneSpec with one commented example beat (word-anchored card)

What changed

• scripts/new.mjs — the scaffold script (new file). Validates a three-digit number and kebab-case slug, refuses to clobber existing files, writes the two scaffolds, and prints next steps.
• package.json — registers "new": "node scripts/new.mjs".

Shapes followed

• Post JSON mirrors posts/104-dev-workflow.json (slug, title, tagline, accent, outro, script, fallbackDurationSeconds, claims[]) with empty claims[].
• Scene module mirrors src/scenes/data/097.ts / 104.ts — imports SceneSpec from ../schema, exports scene<number>: SceneSpec with a slug and word-anchored beats.

Gates: The scaffold writes only under posts/ and src/scenes/data/ — no new TS is compiled into the video kit until the creator registers it, so tsc --noEmit and npm test are unaffected. new.mjs isn't part of the TypeScript project (not type-checked). The change awaits a host render/QA only if a scaffolded scene is registered and voiced; this PR adds no runnable video on its own.

Verification: node scripts/new.mjs 109 build-frames was exercised in a sandbox before committing — it writes both files and they match the exemplar shapes. No claims[] were added (this is a tooling change, not a video script).