feat(context): tambah context::window::resolve

Satukan tiga salinan logika resolusi context_window (Action::Compact,
spawn_turn, status bar) yang sempat melenceng satu sama lain.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/runtime/context/mod.rs

@@ -11,3 +11,4 @@
 //! `TurnEvent::Compacted`.
 
 pub mod tokens;
+pub mod window;

src/app/runtime/context/window.rs

@@ -0,0 +1,74 @@
+//! Single source of truth for resolving the active model's context
+//! window size, replacing three copies of the same lookup that had
+//! drifted (`Action::Compact`, `spawn_turn`, and `view/status.rs` each
+//! had their own inline version — the status bar's copy additionally
+//! displayed "?" on no match instead of falling back like the other two,
+//! an inconsistency this unifies away).
+
+use crate::model::app_config::AppConfig;
+use crate::model::settings::Settings;
+
+/// Resolve the context-window size (in tokens) for the currently
+/// configured provider/model.
+///
+/// Flow: find the `ModelRole` whose `provider`+`model` match
+/// `settings` -> use its `context_window` if set -> otherwise fall back
+/// to `app_config.default_context_window`.
+///
+/// Return: always a concrete token count, never "unknown".
+pub fn resolve(app_config: &AppConfig, settings: &Settings) -> usize {
+    app_config.model_roles.values()
+        .find(|role| role.provider == settings.provider && role.model == settings.model)
+        .and_then(|role| role.context_window)
+        .unwrap_or(app_config.default_context_window) as usize
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::model::app_config::ModelRole;
+
+    #[test]
+    fn resolves_context_window_from_matching_model_role() {
+        let mut app_config = AppConfig::default();
+        app_config.model_roles.insert("default".to_string(), ModelRole {
+            provider: "zen".to_string(),
+            model: "deepseek-v4-flash-free".to_string(),
+            max_tokens: None,
+            context_window: Some(128_000),
+            temperature: None,
+        });
+        let mut settings = Settings::default();
+        settings.provider = "zen".to_string();
+        settings.model = "deepseek-v4-flash-free".to_string();
+
+        assert_eq!(resolve(&app_config, &settings), 128_000);
+    }
+
+    #[test]
+    fn falls_back_to_default_context_window_when_no_role_matches() {
+        let app_config = AppConfig::default();
+        let mut settings = Settings::default();
+        settings.provider = "nonexistent".to_string();
+        settings.model = "nonexistent-model".to_string();
+
+        assert_eq!(resolve(&app_config, &settings), app_config.default_context_window as usize);
+    }
+
+    #[test]
+    fn falls_back_to_default_when_matching_role_has_no_context_window_set() {
+        let mut app_config = AppConfig::default();
+        app_config.model_roles.insert("default".to_string(), ModelRole {
+            provider: "zen".to_string(),
+            model: "deepseek-v4-flash-free".to_string(),
+            max_tokens: None,
+            context_window: None,
+            temperature: None,
+        });
+        let mut settings = Settings::default();
+        settings.provider = "zen".to_string();
+        settings.model = "deepseek-v4-flash-free".to_string();
+
+        assert_eq!(resolve(&app_config, &settings), app_config.default_context_window as usize);
+    }
+}