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
         }