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/map_std.nyash

@@ -0,0 +1,12 @@
+// nyashstd.map – P0 scaffold (JIT-only)
+// Minimal placeholder to define file layout and ny_plugins mapping.
+
+static box StdMap {
+  init { }
+
+  len(m) { return 0 }
+  get(m, k) { return 0 }
+  set(m, k, v) { return 0 }
+  keys(m) { return 0 }
+}
+