feat: add relay-broker Rust binary from PR #404

Pull relay-broker directory from relay-broker-integration branch as the
foundation for the hybrid approach. Will apply bug fixes from PR #412 on top.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: khaliqgant

relay-broker/Cargo.lock

@@ -0,0 +1,41 @@
+[package]
+name = "relay-broker"
+version = "0.1.0"
+edition = "2021"
+autobins = false
+
+[lib]
+name = "relay_broker"
+path = "src/lib.rs"
+
+[[bin]]
+name = "agent-relay"
+path = "src/main.rs"
+
+[dependencies]
+anyhow = "1.0"
+axum = "0.8"
+chrono = { version = "0.4", features = ["serde", "clock"] }
+clap = { version = "4.5", features = ["derive"] }
+dirs = "6.0"
+futures = "0.3"
+parking_lot = "0.12"
+portable-pty = "0.8"
+rand = "0.8"
+regex = "1.11"
+reqwest = { version = "0.12", features = ["json", "rustls-tls"] }
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+thiserror = "2.0"
+tokio = { version = "1.44", features = ["full"] }
+tokio-tungstenite = { version = "0.24", features = ["rustls-tls-native-roots"] }
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter", "fmt"] }
+uuid = { version = "1.15", features = ["v4", "serde"] }
+
+[target.'cfg(unix)'.dependencies]
+nix = { version = "0.30", features = ["signal", "process", "term"] }
+
+[dev-dependencies]
+httpmock = "0.7"
+tempfile = "3.19"

relay-broker/Cargo.toml

@@ -0,0 +1,37 @@
+# Agent Relay Protocol
+
+IMPORTANT: Do NOT use built-in agent tools (Task, SendMessage, TeamCreate, or any native agent spawning) for agent communication. ALWAYS use the relay protocol described below. This applies to spawning agents, sending messages, and all inter-agent coordination. The relay protocol is the ONLY supported method for agent-to-agent communication in this environment.
+
+When you receive a line like `Relay message from <agent> [<id>]: ...`, treat it as an inbound relay message and reply using one of the methods below instead of plain terminal text.
+
+## Sending Messages
+
+### Option 1: MCP tools (preferred)
+
+Use the Relaycast MCP tools to send messages. First call `set_workspace_key` with the value of your `RELAY_API_KEY` environment variable, then `register` as an agent, then send:
+
+- `post_message(channel, text)` — post to a channel
+- `send_dm(to, text)` — DM an agent
+- `check_inbox()` — check for unread messages
+- `list_agents()` — list agents in the workspace
+
+### Option 2: CLI (fallback)
+
+`relay_send` is a shell command available on PATH:
+
+```bash
+relay_send <agent-name> "your message"    # DM to an agent
+relay_send '#channel'   "your message"    # post to a channel
+```
+
+Note: the CLI may fail in sandboxed environments that block network access.
+
+To control the broker daemon, send JSON command messages to the lead agent:
+
+```json
+{"broker":"agent-relay","kind":"spawn","name":"Worker1","cli":"codex"}
+```
+
+```json
+{"broker":"agent-relay","kind":"release","name":"Worker1"}
+```

relay-broker/relay-snippets/agent-relay-snippet.md

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+{
+  "mcpServers": {
+    "relaycast": {
+      "args": [
+        "-y",
+        "@relaycast/mcp"
+      ],
+      "command": "npx",
+      "env": {
+        "RELAY_AGENT_NAME": "broker",
+        "RELAY_API_KEY": "rk_live_b00fd0ddeca96468d72140c9c4b3a910",
+        "RELAY_BASE_URL": "https://api.relaycast.dev"
+      }
+    }
+  }
+}

relay-broker/scripts/multi-review/.mcp.json

@@ -0,0 +1,11 @@
+{
+  "name": "multi-review",
+  "version": "1.0.0",
+  "type": "module",
+  "scripts": {
+    "start": "node review.mjs"
+  },
+  "dependencies": {
+    "@agent-relay/sdk-ts": "file:../../packages/sdk-ts"
+  }
+}