fix: bound and complete console relay teardown, restore proxy_failed audit

- Arm the unclaimed-relay watchdog in NewSession rather than Serve, so an
  agent that never opens its ProxyStream is bounded to 10s and reports
  reason "agent_timeout", per the design spec's failure-mode table.
- Session.Close now also closes the accepted net.Conn (stored via setConn),
  so ConsoleProxy.Close() is an unconditional kill of the whole relay chain
  instead of only closing an already-idle listener.
- Emit console.proxy_failed and end the console session from a defer in
  consoleTunnel guarded on relay.Reason(), since guac's OnDisconnect never
  runs when the connect callback errors -- which is the path every relay
  failure this feature introduces takes. Update the two docsite
  troubleshooting rows to match what the audit event can now actually show.

docsite/docs/reference/troubleshooting.md

@@ -86,7 +86,7 @@ instantaneous.
 | Connects, then closes at once | guacd unreachable. Check `GUACD_ADDR` and that the container is running                                         |
 | SSH rejects the key           | The stored key has no private half, or is not on the target                                                     |
 | RDP fails on retry            | Credentials are single-use and consumed at tunnel open enter them again                                         |
-| 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 |
+| Hangs, then disconnects        | The agent never claimed the relay, nothing is listening on the protocol port on the target's own loopback address, or guacd never dialled in time. Check the audit log for `console.proxy_failed` — its reason (`agent_timeout`, `dial_refused`, `guacd_timeout`, `rejected`) names which |
 | Fails only in production      | The reverse proxy is not forwarding WebSocket upgrade headers                                                   |
 
 ## Monitors report down when the service is up