fix(tui): use displayed cost in /cost and /tokens commands

Switch /cost and /tokens from session_cost+subagent_cost (raw sum) to
displayed_session_cost_for_currency so the numbers match the footer and
sidebar UI, which apply the high-water mark for consistency.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/tui/src/commands/debug.rs

@@ -74,8 +74,7 @@ pub fn tokens(app: &mut App) -> CommandResult {
         .replace(
             "{cost}",
             &app.format_cost_amount_precise(
-                app.session_cost_for_currency(app.cost_currency)
-                    + app.subagent_cost_for_currency(app.cost_currency),
+                app.displayed_session_cost_for_currency(app.cost_currency),
             ),
         )
         .replace("{api_messages}", &message_count.to_string())
@@ -86,8 +85,7 @@ pub fn tokens(app: &mut App) -> CommandResult {
 
 /// Show session cost breakdown
 pub fn cost(app: &mut App) -> CommandResult {
-    let total = app.session_cost_for_currency(app.cost_currency)
-        + app.subagent_cost_for_currency(app.cost_currency);
+    let total = app.displayed_session_cost_for_currency(app.cost_currency);
     let report = tr(app.ui_locale, MessageId::CmdCostReport).replace(
         "{cost}",
         &app.format_cost_amount_precise(total),