feat(go): add coverage-summary as a Go bundle coverage slice

Third language in the coverage-summary fan-out, after Elisp and Python. Same kernel: count every source file on disk that's absent from the coverage profile as 0% and weight the project number by file, so an untested file stays visible instead of being averaged away.

The script at languages/go/claude/scripts/coverage-summary.go parses a cover.out profile, maps each import-path-qualified entry back to an on-disk relative path using the module path from go.mod, and reports a file-weighted number plus the missing files. It's standard library only, so it runs anywhere via go run, and it doesn't reimplement the per-function table that go tool cover -func already prints. I proved it against a real go test -coverprofile run, not just a synthetic fixture, since the Go toolchain is installed here.

Two findings to flag. Modern go test ./... already lists every module package in the profile at 0% even when untested, so for in-module code the missing-file list is usually empty. The detection earns its keep on build-tagged files and dirs outside ./.... And this is a coverage-only slice of a Go bundle that doesn't otherwise exist yet: there's no go.md rule file, so sync-language-bundle.sh can't fingerprint it (detection keys on a bundle's own .claude/rules). The script installs via make install-lang LANG=go but won't be sync-maintained until the Go bundle gets real rules and a CLAUDE.md. Building that out is the natural companion task.

Tests are black-box: a Go test in its own throwaway module runs the script via go run against temp fixtures and checks output, so the shipped script dir stays test-free. They cover missing-file detection, all-tracked, _test.go exclusion, and the missing-report error. make test gained a go test discovery path for languages/*/tests, guarded so environments without Go skip it cleanly.

1 files changed, 43 insertions, 0 deletions

diff --git a/languages/go/coverage-makefile.txt b/languages/go/coverage-makefile.txt
new file mode 100644
index 0000000..7aba9e9
--- /dev/null
+++ b/languages/go/coverage-makefile.txt
@@ -0,0 +1,43 @@
+# Go coverage — Makefile fragment + setup recommendation
+#
+# This file is owned by the project, not the rulesets bundle. The bundle never
+# edits your Makefile. Copy the two targets below into your own Makefile (and
+# adjust the variables at the top), then delete this file or keep it as a note.
+#
+# What you get:
+#   make coverage          runs the suite with a coverage profile and prints
+#                          `go tool cover -func`'s own per-function table
+#   make coverage-summary  prints a file-weighted project number and every
+#                          source file on disk absent from the profile, at 0%.
+#
+# Why the summary matters: it weights by file and counts a file absent from the
+# profile as 0%, so an untested file stays visible instead of being averaged
+# away. Note that modern `go test ./...` already lists every module package in
+# the profile (at 0% when untested), so for in-module code the missing-file list
+# is usually empty; it earns its keep on files outside the test compilation
+# (build-tagged files, a dir not covered by ./...). The summary does not
+# reimplement the per-function table — `go tool cover -func` already prints it.
+#
+# ---------------------------------------------------------------------------
+# Prerequisite: none beyond the Go toolchain. `go test -coverprofile` and
+# `go tool cover` ship with Go; the summary script is pure standard library.
+# ---------------------------------------------------------------------------
+
+# Variables — adjust to your layout.
+GO               ?= go
+SOURCE_DIR       ?= .
+COVERAGE_FILE    ?= cover.out
+# The summary script ships with the bundle under .claude/scripts/ (gitignored).
+COVERAGE_SUMMARY ?= .claude/scripts/coverage-summary.go
+
+coverage:
+	@$(GO) test ./... -coverprofile=$(COVERAGE_FILE)
+	@$(GO) tool cover -func=$(COVERAGE_FILE)
+	@$(MAKE) coverage-summary
+
+coverage-summary:
+	@if [ ! -f $(COVERAGE_FILE) ]; then \
+		echo "[!] No coverage file at $(COVERAGE_FILE). Run 'make coverage' first."; \
+		exit 1; \
+	fi
+	@$(GO) run $(COVERAGE_SUMMARY) $(COVERAGE_FILE) $(SOURCE_DIR) .