Add insert command

internal/insert/conflicts.go

@@ -0,0 +1,32 @@
+package insert
+
+import (
+	"fmt"
+)
+
+// ConflictDetector tracks proposed targets to detect duplicates.
+type ConflictDetector struct {
+	planned map[string]string // proposedRelative -> originalRelative
+}
+
+// NewConflictDetector creates an empty detector.
+func NewConflictDetector() *ConflictDetector {
+	return &ConflictDetector{planned: make(map[string]string)}
+}
+
+// Register validates the proposed target and returns an error string if conflict occurred.
+func (d *ConflictDetector) Register(original, proposed string) (string, bool) {
+	if proposed == "" {
+		return "", false
+	}
+	if existing, ok := d.planned[proposed]; ok && existing != original {
+		return fmt.Sprintf("duplicate target with %s", existing), false
+	}
+	d.planned[proposed] = original
+	return "", true
+}
+
+// Forget removes a planned target (used if operation skipped).
+func (d *ConflictDetector) Forget(proposed string) {
+	delete(d.planned, proposed)
+}