fix(media-use): fall back to bundled SFX

[miguel-heygen]

What

Make media-use resolve --type sfx fall back to the shipped SFX library when the HeyGen CLI/catalog is unavailable. The remote catalog remains the first choice.

Why

CLI 0.7.53 documents a bundled 19-file SFX fallback, but the resolver registry exposed only heygen.audio.sounds. On a machine without the HeyGen CLI, even --intent whoosh failed with no provider could resolve although whoosh.mp3 was installed with the skill.

How

• Add a deterministic bundled-SFX provider backed by audio/assets/sfx/manifest.json.
• Register it after the HeyGen catalog as a local/free fallback.
• Match exact effect names first, then deterministic intent-token overlap.
• Add registry and end-to-end coverage that removes HeyGen from PATH and verifies the frozen local asset.
• Regenerate the skills manifest.

Test plan

• bun run test:skills (219 passed)
• node --test skills/media-use/scripts/lib/registry.test.mjs skills/media-use/scripts/resolve.test.mjs
• npx oxfmt --check skills/media-use/scripts/lib/bundled-sfx-provider.mjs skills/media-use/scripts/lib/registry.mjs skills/media-use/scripts/lib/registry.test.mjs skills/media-use/scripts/resolve.test.mjs
• npx oxlint skills/media-use/scripts/lib/bundled-sfx-provider.mjs skills/media-use/scripts/lib/registry.mjs skills/media-use/scripts/lib/registry.test.mjs skills/media-use/scripts/resolve.test.mjs
• bun packages/cli/scripts/gen-skills-manifest.ts --check