Map lib/src/remote/direct and the sidecar's node-datachannel addon in AGENTS.md

[dormouse-bot]

lib/src/remote/direct/ has been the shared half of the WebRTC direct path since #613, and AGENTS.md's architecture map does not mention it. An agent navigating the repo from that map — which is what the map is for — sees burrow/, client/, pocket-app/, and ws.ts under lib/src/remote/ and has no reason to look for the ~1,900 lines that implement the cutover, the peer wrapper, and the per-session endpoint both ends run.

Two lines change:

• lib/src/remote/ gains direct/, described as the direct path both ends run (the endpoint, peer, and cutover are one implementation shared by the Client and the Burrow, which is the fact the map should carry).
• standalone/sidecar/ now says node-datachannel beside node-pty. The sidecar picked up a second native addon in the same change — standalone/sidecar/package.json declares it and six prebuilt platform packages — so "Node.js PTY manager (native PTY via node-pty)" is no longer the whole of what ships there.

Documentation only, so there is no regression test. node scripts/spec-lint.mjs passes on the branch, including the path-existence check on the new lib/src/remote/direct/ reference; AGENTS.md lands at 3341 words against its 3350 budget, so scripts/spec-word-budgets.json is unchanged.

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