chore: move default ports out of common-collision ranges

Previous defaults collided with well-trodden infra services and with
the Linux ephemeral port range:

- cortex API     8000 — common dev-server default (Django, minio UI)
- cortex metrics 9100 — Prometheus node_exporter default
- neuron API     9090 — Cockpit default on Fedora, Prometheus self

Move to helexa-themed palindromic ports, all below Linux's
32768-60999 ephemeral range and not registered to any well-known
service:

- cortex API     31313
- cortex metrics 31314
- neuron API     13131

Updated places:
- cortex.example.toml, neuron.example.toml defaults
- default impls in cortex-core and neuron config
- cortex-cli --endpoint default for the status subcommand
- doc comments citing example URLs
- README.md and CLAUDE.md snippets

Consumers already on the old ports need a one-line edit in their
/etc/cortex/cortex.toml or /etc/neuron/neuron.toml to match;
firewall rules and prometheus scrape configs will also need
updating.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -88,8 +88,8 @@ WantedBy=multi-user.target
 ```toml
 # cortex.toml
 [gateway]
-listen = "0.0.0.0:8000"
-metrics_listen = "0.0.0.0:9100"
+listen = "0.0.0.0:31313"
+metrics_listen = "0.0.0.0:31314"
 
 [eviction]
 strategy = "lru"        # lru | priority
@@ -143,7 +143,7 @@ cortex serve --config cortex.toml
 cortex status
 
 # list all models across nodes
-curl http://localhost:8000/v1/models
+curl http://localhost:31313/v1/models
 ```
 
 ## License