chore(makefile): pull legacy version sentinel into a make variable

Author: Brad Kinnard

Makefile

@@ -1,5 +1,9 @@
 .PHONY: regen-self-host-fixtures verify-release
 
+# Pre-1.0 sentinel version. Stale references to this string must not appear in
+# shipped files; update only if a new major version creates a new legacy line.
+LEGACY_VERSION := 0.2.0
+
 regen-self-host-fixtures:
 	python3 scripts/regen_self_host_fixtures.py
 
@@ -8,6 +12,6 @@ verify-release:
 	skillcheck --version
 	skillcheck skills/skillcheck/SKILL.md
 	skillcheck skills/skillcheck/SKILL.md --analyze-graph
-	grep -rn "0\.2\.0" --include="*.py" --include="*.toml" --include="*.md" --include="*.yml" src/ pyproject.toml README.md action.yml && echo "FAIL: 0.2.0 references found" && exit 1 || echo "OK: no 0.2.0 references in release files"
+	grep -rn "$(LEGACY_VERSION)" --include="*.py" --include="*.toml" --include="*.md" --include="*.yml" src/ pyproject.toml README.md action.yml && echo "FAIL: $(LEGACY_VERSION) references found" && exit 1 || echo "OK: no $(LEGACY_VERSION) references in release files"
 	grep -rn "moonrunnerkc/skillcheck@v0" README.md && echo "FAIL: @v0 reference in README" && exit 1 || echo "OK: no @v0 in README"
 	@command -v python3 -c "import build" 2>/dev/null && python3 -m build --sdist --wheel || echo "INFO: build module not available, skipping sdist/wheel check"