Cache download-files in Dockerfile and simplify the download-files script

[aryeila]

Overview

Two related cleanups around download-files:

1. Move the Dockerfile download-files step before COPY . . so the download layer caches across code-only changes — editing your agent no longer triggers a re-download of plugin model files.
2. Drop the redundant pnpm run build && prefix from the download-files script. The standalone npx livekit-agents download-files command (already referenced by this script) discovers installed @livekit/agents-plugin-* packages without loading your agent code, so the build step before it was a no-op.

Changes

• Dockerfile — RUN pnpm download-files moved before COPY . . (was previously between pnpm build and pnpm prune --prod).
• package.json — "download-files": "pnpm run build && npx livekit-agents download-files" → "download-files": "npx livekit-agents download-files".

@livekit/agents is pinned at ^1.4.3, which is the version that introduced the standalone livekit-agents download-files CLI.

Related PRs

This is part of a coordinated rollout across four repos. Each PR updates one slice of the same migration.

• Docs: livekit/web#4545
• CLI templates: Cache download-files in Dockerfile templates by using standalone commands livekit/livekit-cli#848
• Python starter: Cache download-files in Dockerfile by using the module-level command agent-starter-python#79
• Node.js starter (this PR): Cache download-files in Dockerfile and simplify the download-files script #49

Upstream SDK changes:

• Python: feat: support python -m livekit.agents.download for asset fetching livekit/agents#5738 (livekit-agents@1.5.10)
• Node.js: feat: livekit-agents download-files script to optimize Docker layers livekit/agents-js#1511 (@livekit/agents@1.4.3)