feat: emberd in-VM daemon for structured RPC over vsock

[jasonhernandez]

Summary

Add emberd, a lightweight daemon that runs inside Ember VMs and exposes structured RPC over vsock (or UDS for testing). Replaces ad-hoc SSH commands with a proper API.

• New crate: emberd/ — Rust binary, ~500 lines
• Protocol: JSON-lines over vsock port 1024 (Linux) or UDS (testing)
• Operations: ping, exec, read_file, write_file, agent_status
• Image integration: emberd binary + systemd unit baked into agent VM images
• Cross-compilation: Docker-based Linux build from macOS (make emberd-image)

Design

• Thread-per-connection model (simple, appropriate for VM-local traffic)
• Linux: AF_VSOCK listener via nix crate
• macOS: UDS-only mode for testing (--uds /path/to/socket)
• Falls back gracefully — if emberd isn't running, Thermite uses SSH

Files changed

File	What
emberd/Cargo.toml	New crate
emberd/src/main.rs	Daemon implementation
emberd/README.md	Protocol docs
images/Dockerfile.ubuntu-dev	Install emberd + systemd unit
images/Dockerfile.ubuntu-slim	Install emberd + systemd unit
images/emberd.service	systemd unit file
Cargo.toml	Add emberd to workspace
Makefile	make emberd, make emberd-image targets

Independent of the vsock PR (#8) — emberd can be reviewed and merged separately.

Test plan

• cargo build --workspace clean
• cargo clippy --workspace clean
• cargo build -p emberd produces working binary
• Manual: run emberd --uds /tmp/test.sock, connect, send {"op":"ping"}

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