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/usage/makefile.md

@@ -0,0 +1,92 @@
+# Makefile
+
+The root `Makefile` provides a single entry point for all common development
+tasks. Targets are namespaced by language (`go-*`, `py-*` when the Python port
+lands) with short umbrella targets (`build`, `test`, …) that delegate to the
+language-specific ones.
+
+## Quick reference
+
+```sh
+make              # same as: make help
+make help         # list all targets with descriptions
+```
+
+## Umbrella targets
+
+| Target | Description |
+|--------|-------------|
+| `all` | Run `check` then `build` |
+| `build` | Build the binary (→ `go-build`) |
+| `test` | Run all tests (→ `go-test`) |
+| `check` | Pre-commit gate: fmt + vet + test (→ `go-check`) |
+| `fmt` | Format source (→ `go-fmt`) |
+| `vet` | Static analysis (→ `go-vet`) |
+| `clean` | Remove binary and coverage artifacts |
+
+## Go targets
+
+| Target | Description |
+|--------|-------------|
+| `go-build` | Compile `go/latprobe` |
+| `go-run` | Build and run (pass flags via `ARGS=`) |
+| `go-test` | `go test ./...` |
+| `go-test-verbose` | `go test -v ./...` — per-case PASS/FAIL output |
+| `go-check` | `go-fmt` + `go-vet` + `go-test` |
+| `go-cover` | Coverage report → `go/coverage.html` |
+| `go-fmt` | `gofmt -w go/` |
+| `go-vet` | `go vet ./...` |
+| `go-tidy` | `go mod tidy` |
+| `go-install` | Install binary to `$GOBIN` |
+| `go-lint` | `golangci-lint run` (must be installed) |
+| `go-clean` | Remove `go/latprobe`, `go/coverage.out`, `go/coverage.html` |
+
+## Examples
+
+```sh
+# Build and run a quick probe
+make go-run ARGS="https://example.com"
+make go-run ARGS="-n 5 --json https://example.com https://www.google.com"
+
+# Run the full test suite
+make test
+
+# Verbose output showing each test case
+make go-test-verbose
+
+# Pre-commit gate (format, vet, test all in one)
+make check
+
+# Generate and open coverage report
+make go-cover
+open go/coverage.html    # macOS
+
+# Lint
+make go-lint
+
+# Clean up artifacts
+make clean
+```
+
+## Passing arguments to `go-run`
+
+```sh
+make go-run ARGS="<url> [url ...] [flags]"
+
+# Examples
+make go-run ARGS="https://example.com"
+make go-run ARGS="--timeout 2s --fail https://example.com"
+make go-run ARGS="--json -n 3 https://example.com https://www.google.com"
+```
+
+## Adding Python targets (when the port lands)
+
+The umbrella targets are designed to accept Python targets alongside the Go ones.
+The pattern will be:
+
+```make
+test: go-test py-test   # py-test added here during the port
+```
+
+`go-lint` requires `golangci-lint` to be installed. If it's missing the target
+prints an install link and exits 1.