feat: replace neko with native GStreamer WebRTC for desktop streaming

Replace the neko binary dependency with a native GStreamer pipeline
(ximagesrc -> vp8enc -> webrtcbin) for desktop video streaming. This
removes the external neko process and integrates screen capture directly
via gstreamer-rs crate bindings behind a `desktop-gstreamer` feature flag.

Key changes:
- Add desktop_gstreamer.rs with GStreamer WebRTC pipeline management
- Rewrite signaling protocol (ready/offer/answer/candidate over WS)
- Add leaky queues and videorate for low-latency streaming
- Rewrite ICE candidates to 127.0.0.1 for Docker connectivity
- Constrain UDP port range (30000-30100) via libnice agent
- Update TypeScript SDK desktop-stream.ts for new signaling
- Update inspector DesktopTab with WebRTC Live View
- Update Dockerfiles to install GStreamer dev packages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/test-agent/Dockerfile

@@ -1,6 +1,16 @@
 FROM rust:1.88.0-bookworm AS builder
 WORKDIR /build
 
+# Install GStreamer dev packages for the desktop-gstreamer feature.
+RUN apt-get update -qq && \
+    apt-get install -y -qq --no-install-recommends \
+      libgstreamer1.0-dev \
+      libgstreamer-plugins-base1.0-dev \
+      libgstreamer-plugins-bad1.0-dev \
+      libnice-dev \
+      > /dev/null 2>&1 && \
+    rm -rf /var/lib/apt/lists/*
+
 COPY Cargo.toml Cargo.lock ./
 COPY server/ ./server/
 COPY gigacode/ ./gigacode/
@@ -12,7 +22,7 @@ ENV SANDBOX_AGENT_SKIP_INSPECTOR=1
 RUN --mount=type=cache,target=/usr/local/cargo/registry \
     --mount=type=cache,target=/usr/local/cargo/git \
     --mount=type=cache,target=/build/target \
-    cargo build -p sandbox-agent --release && \
+    cargo build -p sandbox-agent --release --features desktop-gstreamer && \
     cp target/release/sandbox-agent /sandbox-agent
 
 FROM node:22-bookworm-slim
@@ -26,6 +36,15 @@ RUN apt-get update -qq && \
       xdotool \
       imagemagick \
       ffmpeg \
+      gstreamer1.0-tools \
+      gstreamer1.0-plugins-base \
+      gstreamer1.0-plugins-good \
+      gstreamer1.0-plugins-bad \
+      gstreamer1.0-plugins-ugly \
+      gstreamer1.0-nice \
+      gstreamer1.0-x \
+      gstreamer1.0-pulseaudio \
+      libxcvt0 \
       x11-xserver-utils \
       dbus-x11 \
       xauth \