Step 28: Machine identity and targeting core.

Entry gains Only/Never fields for per-machine targeting. Machine
identity = short hostname + os:<GOOS> + arch:<GOARCH> + tag:<name>.
Tags stored in local <repo>/tags file (added to .gitignore by init).
EntryApplies() matching: only=any-match, never=no-match, both=error.
13 tests covering matching, identity, tags CRUD, gitignore.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

garden/targeting.go

@@ -0,0 +1,48 @@
+package garden
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/kisom/sgard/manifest"
+)
+
+// EntryApplies reports whether the given entry should be active on a
+// machine with the given labels. Returns an error if both Only and
+// Never are set on the same entry.
+func EntryApplies(entry *manifest.Entry, labels []string) (bool, error) {
+	if len(entry.Only) > 0 && len(entry.Never) > 0 {
+		return false, fmt.Errorf("entry %s has both only and never set", entry.Path)
+	}
+
+	if len(entry.Only) > 0 {
+		for _, matcher := range entry.Only {
+			if matchesLabel(matcher, labels) {
+				return true, nil
+			}
+		}
+		return false, nil
+	}
+
+	if len(entry.Never) > 0 {
+		for _, matcher := range entry.Never {
+			if matchesLabel(matcher, labels) {
+				return false, nil
+			}
+		}
+	}
+
+	return true, nil
+}
+
+// matchesLabel checks if a matcher string matches any label in the set.
+// Matching is case-insensitive.
+func matchesLabel(matcher string, labels []string) bool {
+	matcher = strings.ToLower(matcher)
+	for _, label := range labels {
+		if strings.ToLower(label) == matcher {
+			return true
+		}
+	}
+	return false
+}