Unblock build-fast from native WebRTC metadata fetch

[cbusillo]

Summary

• remove the default macOS native WebRTC libwebrtc git dependency from code-rs/realtime-webrtc so Cargo metadata no longer cold-fetches juberti-oai/rust-sdks and livekit/protocol before builds
• keep the TUI-facing WebRTC API surface as an unsupported stub; websocket realtime remains the supported default transport
• document the temporary native WebRTC overlay boundary in code-rs/realtime-webrtc/README.md
• update the new exec harness safety scenarios for the current item.started + item.completed command lifecycle emitted by a freshly built binary

Refs #396

Validation

• CARGO_HOME=/Users/cbusillo/Developer/code/.cargo-home RUSTUP_HOME=/Users/cbusillo/Developer/code/.cargo-home/rustup ./build-fast.sh
• just harness-smoke
• scoped Claude review: no blocking findings

Notes

• WebRTC realtime transport now fails clearly with UnsupportedPlatform in the default build. Restoring native WebRTC should be a deliberate overlay with an opt-in dependency path, not a default build dependency that can stall local dogfood validation.