feat: phase 2 — coûts par session (stats --costs) + am monitor TUI (closes #49 #50)

src/config.rs

@@ -124,6 +124,43 @@ pub struct Settings {
     /// Purge sessions finished more than N days ago (issue #53, default 90).
     #[serde(default)]
     pub sessions_retention_days: Option<u64>,
+    /// Token price models used to estimate session costs (issue #49).
+    #[serde(default)]
+    pub cost_models: Option<BTreeMap<String, CostModel>>,
+    /// CPU/memory thresholds for 'am monitor' alerts (issue #50).
+    #[serde(default)]
+    pub monitor_thresholds: Option<MonitorThresholds>,
+}
+
+/// Token price model: USD per million tokens (issue #49).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct CostModel {
+    pub input_per_mtok: f64,
+    pub output_per_mtok: f64,
+}
+
+/// Default price sheet (USD / MTok) used when settings.cost_models is absent.
+pub fn default_cost_models() -> BTreeMap<String, CostModel> {
+    let mut m = BTreeMap::new();
+    m.insert("claude-sonnet-4".into(), CostModel { input_per_mtok: 3.0, output_per_mtok: 15.0 });
+    m.insert("claude-opus-4".into(), CostModel { input_per_mtok: 15.0, output_per_mtok: 75.0 });
+    m.insert("claude-haiku-3.5".into(), CostModel { input_per_mtok: 0.8, output_per_mtok: 4.0 });
+    m.insert("gpt-4o".into(), CostModel { input_per_mtok: 2.5, output_per_mtok: 10.0 });
+    m.insert("gpt-4o-mini".into(), CostModel { input_per_mtok: 0.15, output_per_mtok: 0.6 });
+    m.insert("deepseek-chat".into(), CostModel { input_per_mtok: 0.27, output_per_mtok: 1.10 });
+    m.insert("llama-3.1-8b".into(), CostModel { input_per_mtok: 0.05, output_per_mtok: 0.05 });
+    m
+}
+
+/// Alert thresholds for 'am monitor' (issue #50).
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct MonitorThresholds {
+    /// Alert when a managed process exceeds this CPU % (0 = disabled).
+    #[serde(default)]
+    pub cpu_pct: Option<f64>,
+    /// Alert when a managed process exceeds this RSS in MB (0 = disabled).
+    #[serde(default)]
+    pub mem_mb: Option<u64>,
 }
 
 /// Policy controlling how aggressively 'am update' upgrades installed agents.
@@ -250,6 +287,10 @@ pub struct AgentDef {
     /// Container profile used by 'am run --container' (issue #58).
     #[serde(default)]
     pub container: Option<ContainerSpec>,
+    /// Price model key (settings.cost_models) used to estimate session costs
+    /// (issue #49; defaults to the first model of the price sheet).
+    #[serde(default)]
+    pub cost_model: Option<String>,
 }
 
 /// Health check for group orchestration (issue #57): a command that must