fix(cli): wire /tokens and /cache as aliases for /stats; implement /stats

Dogfood found that /tokens and /cache had spec entries (resume_supported:
true) but no parse arms in the command parser, resulting in:
  'Unknown slash command: /tokens — Did you mean /tokens'
(the suggestion engine found the spec entry but parsing always failed)

Fix three things:
1. Add 'tokens' | 'cache' as aliases for 'stats' in the parse match so
   the commands actually resolve to SlashCommand::Stats
2. Implement SlashCommand::Stats in the REPL dispatch — previously fell
   through to 'Command registered but not yet implemented'. Now shows
   cumulative token usage for the session.
3. Implement SlashCommand::Stats in run_resume_command — previously
   returned 'unsupported resumed slash command'. Now emits:
   text:  Cost / Input tokens / Output tokens / Cache create / Cache read
   json:  {kind:stats, input_tokens, output_tokens, cache_*, total_tokens}

159 CLI tests pass, fmt clean.

rust/crates/commands/src/lib.rs

@@ -1340,7 +1340,7 @@ pub fn validate_slash_command_input(
             validate_no_args(command, &args)?;
             SlashCommand::Upgrade
         }
-        "stats" => {
+        "stats" | "tokens" | "cache" => {
             validate_no_args(command, &args)?;
             SlashCommand::Stats
         }

rust/crates/rusty-claude-cli/src/main.rs

@@ -2811,6 +2811,21 @@ fn run_resume_command(
             message: Some(render_doctor_report()?.render()),
             json: None,
         }),
+        SlashCommand::Stats => {
+            let usage = UsageTracker::from_session(session).cumulative_usage();
+            Ok(ResumeCommandOutcome {
+                session: session.clone(),
+                message: Some(format_cost_report(usage)),
+                json: Some(serde_json::json!({
+                    "kind": "stats",
+                    "input_tokens": usage.input_tokens,
+                    "output_tokens": usage.output_tokens,
+                    "cache_creation_input_tokens": usage.cache_creation_input_tokens,
+                    "cache_read_input_tokens": usage.cache_read_input_tokens,
+                    "total_tokens": usage.total_tokens(),
+                })),
+            })
+        }
         SlashCommand::History { count } => {
             let limit = parse_history_count(count.as_deref())
                 .map_err(|error| -> Box<dyn std::error::Error> { error.into() })?;
@@ -2847,7 +2862,6 @@ fn run_resume_command(
         | SlashCommand::Logout
         | SlashCommand::Vim
         | SlashCommand::Upgrade
-        | SlashCommand::Stats
         | SlashCommand::Share
         | SlashCommand::Feedback
         | SlashCommand::Files
@@ -3847,11 +3861,15 @@ impl LiveCli {
                 self.print_prompt_history(count.as_deref());
                 false
             }
+            SlashCommand::Stats => {
+                let usage = UsageTracker::from_session(self.runtime.session()).cumulative_usage();
+                println!("{}", format_cost_report(usage));
+                false
+            }
             SlashCommand::Login
             | SlashCommand::Logout
             | SlashCommand::Vim
             | SlashCommand::Upgrade
-            | SlashCommand::Stats
             | SlashCommand::Share
             | SlashCommand::Feedback
             | SlashCommand::Files