feat(agent): read available models from HIVE_AVAILABLE_MODELS env var

web_ui.rs: add available_models() helper that reads HIVE_AVAILABLE_MODELS
(comma-separated, injected by services.hyperhive.availableModels nix option).
Falls back to ["haiku", "sonnet", "opus"] when absent or empty. Field added
to StateSnapshot so the frontend receives the list on cold-load.

app.js: replace hardcoded MODEL_ALIASES array with availableModels module var.
Seeded from state.available_models before setHeader/populateOverflowMenu on
first /api/state load. Well-known aliases (haiku/sonnet/opus) still get their
descriptive labels; operator-declared custom model names show the name itself.

Implements the frontend + web_ui.rs side of issue #1359 (nix option shipped
separately in PR #1360 by atlas).

hive-ag3nt/src/web_ui.rs

@@ -436,6 +436,14 @@ struct StateSnapshot {
     /// Human name of the swarm (e.g. `"constellat1on"`). Sourced from
     /// `HYPERHIVE_SWARM_NAME`; `None` when unset.
     swarm_name: Option<String>,
+    /// Ordered list of model short-names the operator has declared as
+    /// available on this hive. Sourced from `HIVE_AVAILABLE_MODELS`
+    /// (comma-separated, set by `services.hyperhive.availableModels`).
+    /// Falls back to `["haiku", "sonnet", "opus"]` when the env var is
+    /// absent or empty. The frontend model quick-picker renders one button
+    /// per entry in this list, so operators can add new models or drop
+    /// ones they don't want without touching the frontend code.
+    available_models: Vec<String>,
 }
 
 /// One navigation link in the agent page header row. The same JSON
@@ -561,6 +569,7 @@ async fn api_state(State(state): State<AppState>) -> axum::Json<StateSnapshot> {
             .filter(|s| !s.is_empty()),
         hive_name: crate::identity::hive_name(),
         swarm_name: crate::identity::swarm_name(),
+        available_models: available_models(),
     })
 }
 
@@ -1076,3 +1085,25 @@ fn error_response(message: &str) -> Response {
     // be noise.
     (StatusCode::INTERNAL_SERVER_ERROR, message.to_owned()).into_response()
 }
+
+/// Read `HIVE_AVAILABLE_MODELS` (comma-separated short names injected by
+/// `services.hyperhive.availableModels`) and return the parsed list.
+/// Falls back to `["haiku", "sonnet", "opus"]` when the env var is absent
+/// or resolves to an empty list after trimming.
+fn available_models() -> Vec<String> {
+    const DEFAULT: &[&str] = &["haiku", "sonnet", "opus"];
+    let raw = match std::env::var("HIVE_AVAILABLE_MODELS") {
+        Ok(v) if !v.trim().is_empty() => v,
+        _ => return DEFAULT.iter().map(|s| s.to_string()).collect(),
+    };
+    let models: Vec<String> = raw
+        .split(',')
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty())
+        .collect();
+    if models.is_empty() {
+        DEFAULT.iter().map(|s| s.to_string()).collect()
+    } else {
+        models
+    }
+}