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>

cortex.example.toml

@@ -3,11 +3,11 @@
 # Copy to cortex.toml and adjust for your environment.
 #
 # Environment variable overrides use CORTEX_ prefix with __ separators:
-#   CORTEX_GATEWAY__LISTEN=0.0.0.0:9000
+#   CORTEX_GATEWAY__LISTEN=0.0.0.0:31313
 
 [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"