fix(talking-head): preserve source audio

[miguel-heygen]

What

Preserve the source program audio in the canonical talking-head-recut composition.

Why

The shipped template muted the visual #bg-video but mounted no audio track, then instructed users to render directly. A Windows 0.7.53 user received a valid MP4 with hasAudio:false and had to remux the source audio manually.

How

• Mount the same source MP4 as a direct root #source-audio track with matching timing.
• Keep the visual video muted to avoid double playback.
• Document that the normal render now preserves source audio without a manual remux.
• Add a media-contract regression test for the canonical HTML.
• Regenerate the skills manifest.

Test plan

• bun run test:skills (219 passed)
• node --test skills/talking-head-recut/media-contract.test.mjs
• npx oxfmt --check skills/talking-head-recut/SKILL.md skills/talking-head-recut/media-contract.test.mjs
• npx oxlint skills/talking-head-recut/media-contract.test.mjs
• bun packages/cli/scripts/gen-skills-manifest.ts --check