chore(routing): flip LITELLM_BASE_URL default to https://llm-api.d-ma.be

Follow-up to infra#70. LiteLLM moved off piguard into k3s and the
public llm-api.d-ma.be hostname now upstreams to koala:30401. The
piguard:4000 default in the source bit-rots — works today because
piguard:4000 is still alive during the 7-day soak, breaks the moment
the compose comes down.

Pointing the default at the public hostname survives the cutover
without needing a follow-up. Production deploys via k3s already
override via env (in-cluster Service DNS) so this only affects local
dev shells without LITELLM_BASE_URL set.

- internal/config/routing.go: comment + envOr fallback
- internal/config/routing_test.go: expected value in defaults test
- scripts/smoke-routing.sh: shell default

task check: clean (tests + vet + govulncheck).

scripts/smoke-routing.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 # Boot the routing binary and exercise its four tools against live deps.
 # Skipped when LITELLM_BASE_URL or BRAIN_URL is unreachable.
 
-LITELLM_BASE_URL="${LITELLM_BASE_URL:-http://piguard:4000}"
+LITELLM_BASE_URL="${LITELLM_BASE_URL:-https://llm-api.d-ma.be}"
 BRAIN_URL="${BRAIN_URL:-http://koala:30330}"
 
 if ! curl -sS --max-time 2 "${LITELLM_BASE_URL}/v1/models" >/dev/null 2>&1; then