chore: add knip config for dead-code detection

knip.json declares the real entry points (scripts, tests, Remotion roots)
and the devDeps invoked only as external CLIs (esbuild for build,
agent-browser/remotion via npx) so future scans surface only true
findings.

Add \`npm run knip\` as the canonical invocation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>