docs: document the agent-relayed console proxy

Every console session now rides the agent's outbound gRPC connection
instead of a direct guacd-to-target dial, so it works for servers
behind NAT and now requires a live agent (409 agent_offline
otherwise). Documents PROXY_ADVERTISE_HOST / PROXY_LISTEN_HOST and
corrects reachability claims across the docsite and CLAUDE.md.

docsite/docs/vantage/browser-console.md

@@ -15,17 +15,20 @@ to a **guacd** daemon and manages credentials around it.
 - `guacd` running and reachable from the server. The bundled Compose stack
   includes it; `GUACD_ADDR` defaults to `guacd:4822`.
 - `KEY_ENCRYPTION_KEY` set, since every credential involved is stored encrypted.
-- Network reachability **from the control plane to the target** on the protocol
-  port. This is the one part of Vantage that is not agent-mediated: guacd
-  connects directly, so a machine reachable only by its agent cannot be
-  consoled.
+- The target's **agent must be online**. Console traffic is relayed over the
+  agent's existing outbound connection, so the control plane never needs a route
+  to the server's address — but it does need the agent.
+- No inbound port on the target, beyond what the protocol already listens on
+  locally. A service bound only to `127.0.0.1` works, because the agent dials
+  loopback on the target itself.
 
 ## Opening a session
 
 From a server's page, choose **Console**. Then:
 
 1. The UI calls `POST /api/console/connect`, which mints a **one-time** session
-   token.
+   token. If the target's agent is not connected, this fails immediately with
+   `409 agent_offline` rather than hanging.
 2. The browser opens a WebSocket to `GET /api/console/tunnel` with that token.
 3. The server marks the token consumed atomically, so a second use cannot
    race and proxies the connection to guacd.
@@ -67,4 +70,4 @@ keystroke log. If you need that, it has to come from the target machine.
 | Connects then closes immediately | guacd unreachable check `GUACD_ADDR` and that the container is up               |
 | SSH refuses the key              | The stored key has no private half, or is not in the target's `authorized_keys` |
 | RDP fails on a fresh credential  | Credentials are consumed on open; a retry needs them entered again              |
-| Hangs at connecting              | The control plane cannot reach the target on the protocol port                  |
+| Hangs at connecting              | The agent is offline, or nothing is listening on the protocol port on the target's own loopback address. Check the audit log for `console.proxy_failed` — its reason names which |