feat(repl): REPL Quiet Mode - デバッグログ自動抑制

REPL mode では内部デバッグログを自動抑制:
- src/runner/repl/repl_runner.rs: NYASH_QUIET_INTERNAL_LOGS 設定/解除
- src/mir/builder/decls.rs: NYASH_QUIET_INTERNAL_LOGS チェック追加
- NYASH_CLI_VERBOSE=1 でデバッグログ有効化可能

UX improvement:
  Before: >>> print("Hello")
          [build_static_main_box] Storing fn_body_ast...
          Hello

  After:  >>> print("Hello")
          Hello

Test results:
  ✅ 通常モード: ログ抑制
  ✅ デバッグモード (NYASH_CLI_VERBOSE=1): ログ表示
  ✅ File mode regression: 154/154 pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/decls.rs

@@ -129,10 +129,12 @@ impl super::MirBuilder {
                     }
                 }
                 // Phase 200-C: Store fn_body_ast for inline main() lowering
-                eprintln!(
-                    "[build_static_main_box] Storing fn_body_ast with {} nodes for inline main()",
-                    body.len()
-                );
+                if std::env::var("NYASH_QUIET_INTERNAL_LOGS").ok().as_deref() != Some("1") {
+                    eprintln!(
+                        "[build_static_main_box] Storing fn_body_ast with {} nodes for inline main()",
+                        body.len()
+                    );
+                }
                 self.comp_ctx.fn_body_ast = Some(body.clone());
 
                 // Lower statements in order to preserve def→use

src/runner/repl/repl_runner.rs

@@ -99,6 +99,12 @@ impl ReplRunnerBox {
         use crate::mir::MirCompiler;
         use crate::backend::mir_interpreter::MirInterpreter;
 
+        // REPL mode では内部デバッグログを抑制（NYASH_CLI_VERBOSE=1 で無効化）
+        let verbose = std::env::var("NYASH_CLI_VERBOSE").ok().as_deref() == Some("1");
+        if !verbose {
+            std::env::set_var("NYASH_QUIET_INTERNAL_LOGS", "1");
+        }
+
         // Initialize session on first use
         {
             let mut session_ref = self.session.borrow_mut();
@@ -138,6 +144,12 @@ impl ReplRunnerBox {
 
         // Phase 288 P3: print() output already displayed via ExternCall
         // Expression auto-display deferred to Phase 288.1
+
+        // 評価完了後に環境変数をクリーンアップ
+        if !verbose {
+            std::env::remove_var("NYASH_QUIET_INTERNAL_LOGS");
+        }
+
         Ok(String::new())
     }
 }