feat: Implement foundation layer with domain entities and repository interfaces

- Add complete domain layer: Note, Vault, WikiLink, Tag, Frontmatter, Graph entities
- Implement repository interfaces for data access abstraction
- Create comprehensive configuration system with YAML and env support
- Add CLI entry point with signal handling and graceful shutdown
- Fix mermaid diagram syntax in design.md (array notation)
- Add CLAUDE.md for development guidance

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/domain/tag.go

@@ -0,0 +1,79 @@
+package domain
+
+import (
+	"fmt"
+	"strings"
+)
+
+type Tag struct {
+	name string
+}
+
+func NewTag(name string) (*Tag, error) {
+	if name == "" {
+		return nil, fmt.Errorf("tag name cannot be empty")
+	}
+
+	if !strings.HasPrefix(name, "#") {
+		name = "#" + name
+	}
+
+	if !isValidTagName(name) {
+		return nil, ErrInvalidTag
+	}
+
+	return &Tag{name: name}, nil
+}
+
+func (t *Tag) Name() string {
+	return t.name
+}
+
+func (t *Tag) Parent() *Tag {
+	if !t.IsNested() {
+		return nil
+	}
+
+	lastSlash := strings.LastIndex(t.name, "/")
+	if lastSlash == -1 {
+		return nil
+	}
+
+	parentName := t.name[:lastSlash]
+	parent, _ := NewTag(parentName)
+	return parent
+}
+
+func (t *Tag) IsNested() bool {
+	return strings.Contains(t.name, "/")
+}
+
+func (t *Tag) String() string {
+	return t.name
+}
+
+func isValidTagName(name string) bool {
+	if !strings.HasPrefix(name, "#") {
+		return false
+	}
+
+	if len(name) <= 1 {
+		return false
+	}
+
+	tagContent := name[1:]
+
+	if strings.Contains(tagContent, " ") {
+		return false
+	}
+
+	if strings.HasPrefix(tagContent, "/") || strings.HasSuffix(tagContent, "/") {
+		return false
+	}
+
+	if strings.Contains(tagContent, "//") {
+		return false
+	}
+
+	return true
+}