phase15: update CLAUDE.md with Phase 15 enhancements from AGENTS.md

- Add JIT Self-Host Quickstart section for Phase 15
- Include important flags reference (plugins, parsers, debugging)
- Add Codex async workflow documentation for parallel tasks
- Update test execution with Phase 15 smoke tests
- Improve build time notes (JIT vs LLVM)
- Align with current Phase 15 progress and tooling

🎉 Bootstrap (c0→c1→c1') test confirmed working\!

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

apps/using-e2e/main.nyash

@@ -0,0 +1,9 @@
+// using/nyash.link E2E sample (placeholder)
+static box Main {
+  init { }
+  main(args) {
+    // When using/nyash.link is active, modules can be resolved here.
+    // Placeholder just returns 0 for now.
+    return 0
+  }
+}