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

src/events.rs

@@ -42,6 +42,10 @@ pub enum EventKind {
     Rollback,
     /// Derived index pruned (sessions retention, issue #53).
     Prune,
+    /// Session usage recorded: tokens + estimated cost (issue #49).
+    Cost,
+    /// Threshold alert from 'am monitor' (issue #50).
+    Alert,
 }
 
 impl EventKind {
@@ -64,6 +68,8 @@ impl EventKind {
             EventKind::Backup => "backup",
             EventKind::Rollback => "rollback",
             EventKind::Prune => "prune",
+            EventKind::Cost => "cost",
+            EventKind::Alert => "alert",
         }
     }
 }
@@ -95,6 +101,15 @@ pub struct Event {
     pub exit_code: Option<i32>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub duration_s: Option<u64>,
+    /// Input tokens billed for the session (issue #49).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tokens_in: Option<u64>,
+    /// Output tokens billed for the session (issue #49).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tokens_out: Option<u64>,
+    /// Estimated cost in USD (issue #49).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub cost_usd: Option<f64>,
 }
 
 impl Event {
@@ -113,6 +128,9 @@ impl Event {
             env_keys: Vec::new(),
             exit_code: None,
             duration_s: None,
+            tokens_in: None,
+            tokens_out: None,
+            cost_usd: None,
         }
     }
 
@@ -161,8 +179,21 @@ impl Event {
         self
     }
 
-    pub fn with_duration(mut self, secs: u64) -> Self {
-        self.duration_s = Some(secs);
+    pub fn with_duration(mut self, d: u64) -> Self {
+        self.duration_s = Some(d);
+        self
+    }
+
+    /// Record token usage (issue #49).
+    pub fn with_usage(mut self, tokens_in: u64, tokens_out: u64) -> Self {
+        self.tokens_in = Some(tokens_in);
+        self.tokens_out = Some(tokens_out);
+        self
+    }
+
+    /// Record the estimated cost in USD (issue #49).
+    pub fn with_cost(mut self, usd: f64) -> Self {
+        self.cost_usd = Some(usd);
         self
     }
 }