feat(gateway): add node capability negotiation foundation

README.md

@@ -34,6 +34,7 @@ Self-hosted personal AI assistant with Telegram and Terminal interfaces.
 - **Tailscale Serve**: Auto-expose gateway via Tailscale Serve on daemon start with lifecycle management
 - **DM Pairing Codes**: Allow unknown senders to pair with the bot via time-limited codes across all channels, with SQLite-backed persistence across restarts
 - **Lane Queue**: Per-session FIFO queue serializes concurrent gateway requests
+- **Node Capability Negotiation**: Optional companion-node registration and capability discovery over gateway RPC
 
 ## Quick Start
 
@@ -846,6 +847,24 @@ Runtime session controls from chat commands:
 - `/queue set <mode|cap|overflow|debounce_ms|summarize_overflow> <value>` sets a per-session override.
 - `/queue reset` clears per-session queue overrides.
 
+## Gateway Node Capability Negotiation
+
+Optional gateway surface for companion clients and node-role negotiation:
+
+```yaml
+server:
+  nodes:
+    enabled: true
+    allowed_roles: [companion]
+    feature_gates:
+      ui.canvas: true
+```
+
+Methods:
+- `node.register` registers role + declared capabilities for the current connection.
+- `node.capabilities.get` returns negotiated protocol version and enabled capabilities.
+- `system.capabilities` returns gateway protocol and node policy snapshot.
+
 ## Gateway Request Body Limit
 
 Cap inbound HTTP POST body size (webhooks and Gmail push) to reduce memory-DoS risk.