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

src/commands/stats_cmd.rs

@@ -132,10 +132,23 @@ pub fn report_md(app: &App, days: u32) -> String {
             ));
         }
     }
+    // Issue #49: cost section when usage was recorded.
+    if let Ok(rows) = crate::costs::aggregate(app, Some(&format!("{days}d"))) {
+        if !rows.is_empty() {
+            let t = crate::costs::totals(&rows);
+            md.push_str(&format!(
+                "\n## Coûts estimés\n\n- Sessions facturées : {}\n- Tokens : {} in / {} out\n- Coût estimé : ${:.4}\n",
+                t.sessions, t.tokens_in, t.tokens_out, t.cost_usd
+            ));
+        }
+    }
     md
 }
 
-pub fn run(app: &App, agent: Option<&str>, period: Option<&str>) -> Result<i32> {
+pub fn run(app: &App, agent: Option<&str>, period: Option<&str>, costs: bool) -> Result<i32> {
+    if costs {
+        return run_costs(app, period);
+    }
     let rows = aggregate(app, agent, period)?;
     if app.json() {
         print_json(&rows);
@@ -201,6 +214,40 @@ pub fn run_top(app: &App, period: Option<&str>) -> Result<i32> {
     Ok(0)
 }
 
+/// am stats --costs : token usage and estimated spend (issue #49).
+pub fn run_costs(app: &App, period: Option<&str>) -> Result<i32> {
+    let rows = crate::costs::aggregate(app, period)?;
+    if app.json() {
+        print_json(&rows);
+        return Ok(0);
+    }
+    if rows.is_empty() {
+        app.log
+            .info("no cost recorded yet — agents exposing their usage (claude-code) are priced at session end");
+        return Ok(0);
+    }
+    let total = crate::costs::totals(&rows);
+    let mut table = Table::new(vec!["AGENT", "SESSIONS", "TOKENS IN", "TOKENS OUT", "COST $"]);
+    for r in &rows {
+        table.row(vec![
+            r.agent.clone(),
+            r.sessions.to_string(),
+            r.tokens_in.to_string(),
+            r.tokens_out.to_string(),
+            format!("{:.4}", r.cost_usd),
+        ]);
+    }
+    table.row(vec![
+        "TOTAL".to_string(),
+        total.sessions.to_string(),
+        total.tokens_in.to_string(),
+        total.tokens_out.to_string(),
+        format!("{:.4}", total.cost_usd),
+    ]);
+    print!("{}", table.render());
+    Ok(0)
+}
+
 /// am report : write the markdown digest (issue #37).
 pub fn run_report(
     app: &App,