phase15: update CLAUDE.md and sync with current progress

- Update phase indicator to Phase 15 (Self-Hosting)
- Update documentation links to Phase 15 resources
- Reflect completion of R1-R5 tasks and ongoing work
- Fix CURRENT_TASK.md location to root directory

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

crates/nyash-next/src/main.rs

@@ -0,0 +1,5 @@
+fn main() {
+    env_logger::init();
+    println!("nyash-next: workspace skeleton is ready.");
+}
+