ci: add RPM packaging for cortex and neuron

- cortex.spec: gateway binary, cortex.service systemd unit,
  cortex.toml + models.toml config files
- neuron.spec: neuron binary, neuron.service systemd unit,
  neuron.toml config file
- Parallel CI: srpm-cortex and srpm-neuron jobs build SRPMs
  concurrently, then publish to separate COPR repos
  (helexa/cortex and helexa/neuron)
- Shared cortex user/group across both packages
- Example configs: cortex.example.toml, neuron.example.toml,
  models.example.toml

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

neuron.example.toml

@@ -0,0 +1,16 @@
+# neuron.example.toml — example configuration
+#
+# Copy to /etc/cortex/neuron.toml and adjust for your environment.
+#
+# Environment variable overrides use NEURON_ prefix with __ separators:
+#   NEURON_PORT=9090
+
+port = 9090
+
+# -- Harnesses ---------------------------------------------------------------
+# Each [[harnesses]] entry declares an inference engine managed by neuron.
+
+[[harnesses]]
+name = "mistralrs"
+endpoint = "http://localhost:8080"
+systemd_unit = "mistralrs.service"