feat(cortex-gateway): C3 — propagate vision capabilities through /v1/models

ModelEntry and CortexModelEntry gain a `capabilities: Vec<String>`
field (serde-default for back-compat). The poller copies it verbatim
from each neuron's ModelInfo.capabilities; list_models computes the
union across every node where a model is loaded so a checkpoint loaded
text-only on one neuron and text+vision on another reports both to the
fleet. Catalogue-only and mid-prewarm entries default to empty until
the catalogue gains a capabilities declaration.

Aliases inherit their target's capability union. New gateway test mocks
two nodes with differing capability arrays and asserts the unioned
/v1/models response.

Closes part of #16 (Stage C3).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/src/poller.rs

@@ -107,12 +107,14 @@ async fn poll_neuron(fleet: &CortexState, name: &str, endpoint: &str) {
                             .and_modify(|e| {
                                 e.status = status;
                                 e.vram_estimate_mb = upstream.vram_used_mb;
+                                e.capabilities = upstream.capabilities.clone();
                             })
                             .or_insert_with(|| ModelEntry {
                                 id: upstream.id.clone(),
                                 status,
                                 last_accessed: None,
                                 vram_estimate_mb: upstream.vram_used_mb,
+                                capabilities: upstream.capabilities.clone(),
                             });
                     }