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/std/array_std.nyash

@@ -0,0 +1,12 @@
+// nyashstd.array – P0 scaffold (JIT-only)
+// Minimal placeholder to define file layout and ny_plugins mapping.
+
+static box StdArray {
+  init { }
+
+  len(a) { return 0 }
+  push(a, v) { return 0 }
+  pop(a) { return 0 }
+  slice(a, b, e) { return a }
+}
+