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>

docs/plans/2026-07-01-01-05-makefile.md

@@ -0,0 +1,90 @@
+# Plan: Root Makefile
+
+## Context
+
+The project has grown to a multi-step Go implementation with a test suite, and a
+Python port is planned. Common workflows (build, test, vet, coverage) are
+currently typed by hand with `cd go && go ...`. A root `Makefile` gives a single,
+discoverable entry point for these tasks and a place to hang the future Python
+targets.
+
+Decisions confirmed with the user:
+- Include **all** proposed targets: core set + `cover` + `tidy`/`install` + `lint`.
+- **Namespaced** naming (`go-build`, `go-test`, …) with umbrella targets
+  (`build`, `test`, …) that delegate to the language-specific ones, so `py-*`
+  can slot in cleanly during the Python port.
+
+Environment: GNU Make 3.81, Go 1.26.4, `golangci-lint` installed. Go commands use
+`go -C go …` (Go 1.20+ directory flag) to avoid `cd`.
+
+## File: `Makefile` (repo root)
+
+Variables:
+```make
+GO_DIR  := go
+BINARY  := latprobe
+ARGS    ?=
+.DEFAULT_GOAL := help
+```
+
+### Umbrella targets (delegate now to Go; Python added later)
+| Target | Delegates to | Notes |
+|--------|--------------|-------|
+| `help` | — | Default. Auto-generated from `##` comments. |
+| `build` | `go-build` | `py-build` appended during Python port |
+| `test` | `go-test` | `py-test` appended later |
+| `check` | `go-check` | fmt + vet + test gate |
+| `fmt` | `go-fmt` | |
+| `vet` | `go-vet` | |
+| `clean` | `go-clean` | |
+| `all` | `check build` | |
+
+A comment marks where `py-*` will be added (e.g. `test: go-test  # + py-test later`).
+
+### Go targets
+| Target | Command |
+|--------|---------|
+| `go-build` | `go -C $(GO_DIR) build -o $(BINARY) .` |
+| `go-run` | `go -C $(GO_DIR) run . $(ARGS)` |
+| `go-test` | `go -C $(GO_DIR) test ./...` |
+| `go-test-verbose` | `go -C $(GO_DIR) test -v ./...` |
+| `go-fmt` | `gofmt -w $(GO_DIR)` |
+| `go-vet` | `go -C $(GO_DIR) vet ./...` |
+| `go-check` | depends on `go-fmt go-vet go-test` |
+| `go-cover` | `go -C $(GO_DIR) test -coverprofile=coverage.out ./...` then `go -C $(GO_DIR) tool cover -html=coverage.out -o coverage.html` |
+| `go-tidy` | `go -C $(GO_DIR) mod tidy` |
+| `go-install` | `go -C $(GO_DIR) install .` |
+| `go-lint` | guard for `golangci-lint` presence, then `cd $(GO_DIR) && golangci-lint run` |
+| `go-clean` | `rm -f $(GO_DIR)/$(BINARY) $(GO_DIR)/coverage.out $(GO_DIR)/coverage.html` |
+
+Details:
+- `make run ARGS="https://example.com -n 3"` passes flags through.
+- All targets listed in `.PHONY`.
+- `help` recipe: `grep`/`awk` over `$(MAKEFILE_LIST)` printing `target ## description`
+  (works on GNU Make 3.81).
+- `go-lint` guard:
+  ```make
+  @command -v golangci-lint >/dev/null 2>&1 || { echo "golangci-lint not installed: https://golangci-lint.run"; exit 1; }
+  ```
+
+## Other changes
+- `.gitignore`: add `go/coverage.out` and `go/coverage.html`.
+- `docs/usage/makefile.md`: new user doc — target table + examples.
+- `CHANGELOG.md`: timestamped entry.
+- `README.md`: short "Development" note pointing at `make help`.
+- `docs/plans/2026-07-01-01-05-makefile.md`: copy of this plan.
+
+## Verification
+```sh
+make help            # lists all targets with descriptions
+make build           # produces go/latprobe
+make run ARGS="https://example.com"
+make test            # go test ./...
+make go-test-verbose # per-test PASS/FAIL output
+make check           # fmt + vet + test
+make go-cover        # writes go/coverage.html
+make go-lint         # runs golangci-lint (installed)
+make clean           # removes binary + coverage artifacts
+```
+Confirm `make` with no args shows help, and that `clean` leaves the tree as
+`git status` clean (no tracked files removed).