chore: add root Makefile with namespaced targets

Namespaced go-* targets (build, run, test, test-verbose, check, cover,
fmt, vet, tidy, install, lint, clean) plus umbrella targets that delegate
to them. help is the default. py-* targets will slot in during the Python
port. Updates .gitignore with coverage artifacts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -4,6 +4,16 @@ All completed features are logged here in reverse-chronological order.
 
 ---
 
+## 2026-07-01 01:05 — Root Makefile
+
+- Namespaced targets: `go-build`, `go-run`, `go-test`, `go-test-verbose`, `go-check`, `go-cover`, `go-fmt`, `go-vet`, `go-tidy`, `go-install`, `go-lint`, `go-clean`
+- Umbrella targets (`build`, `test`, `check`, `fmt`, `vet`, `clean`, `all`) delegate to Go now; `py-*` will slot in during the Python port
+- `help` is the default target; auto-generated from `##` comments
+- `go-run` accepts `ARGS=` for passing flags, `go-lint` guards for `golangci-lint`
+- `.gitignore` updated with coverage artifacts
+
+---
+
 ## 2026-07-01 00:49 — Integration tests (Go, Step 6)
 
 - Extracted `run(args, stdout, stderr) int` from `main()` to make the CLI testable in-process