docs(repl): Phase 288 documentation cleanup

Updated documentation to reflect actual implementation status:
- docs/reference/language/repl.md: Corrected examples (variable persistence TODO)
- docs/development/current/main/10-Now.md: Phase 288 completion noted
- docs/development/current/main/30-Backlog.md: Updated with Phase 288.1 tasks
- docs/development/current/main/phases/phase-288/: Added phase documentation

Accurate REPL behavior examples:
- x = 1 works (implicit local)
- print(x) errors (persistence in Phase 288.1)
- Expression auto-display deferred

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/development/current/main/30-Backlog.md

@@ -8,6 +8,11 @@ Related:
 
 ## 直近（JoinIR/selfhost）
 
+- **Phase 288（✅ P0–P3 complete）: REPL mode**
+  - 入口: `docs/development/current/main/phases/phase-288/README.md`
+  - SSOT: `docs/reference/language/repl.md`
+  - 次: Phase 288.1（session persistence / auto-display） `docs/development/current/main/phases/phase-288/P1-INSTRUCTIONS.md`
+
 - **Phase 287（active, stabilization）: ビルド/テスト軽量化（quick が重すぎる）**
   - 目的: `tools/smokes/v2/run.sh --profile quick` を 45秒以内（目安）へ戻し、開発サイクルを軽くする
   - 入口: `docs/development/current/main/phases/phase-287/README.md`