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>

crates/cortex-core/src/node.rs

@@ -6,7 +6,7 @@ use std::collections::HashMap;
 #[derive(Debug, Clone)]
 pub struct NodeState {
     pub name: String,
-    /// Base URL of the neuron daemon (e.g. "http://beast.internal:9090").
+    /// Base URL of the neuron daemon (e.g. "http://beast.internal:13131").
     pub endpoint: String,
     pub healthy: bool,
     pub models: HashMap<String, ModelEntry>,