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/frontmatter.go

@@ -0,0 +1,110 @@
+package domain
+
+import (
+	"fmt"
+	"reflect"
+)
+
+type Frontmatter struct {
+	data map[string]interface{}
+}
+
+func NewFrontmatter() *Frontmatter {
+	return &Frontmatter{
+		data: make(map[string]interface{}),
+	}
+}
+
+func NewFrontmatterFromMap(data map[string]interface{}) *Frontmatter {
+	fm := NewFrontmatter()
+	for k, v := range data {
+		fm.data[k] = v
+	}
+	return fm
+}
+
+func (f *Frontmatter) Get(key string) interface{} {
+	return f.data[key]
+}
+
+func (f *Frontmatter) Set(key string, value interface{}) error {
+	if key == "" {
+		return fmt.Errorf("frontmatter key cannot be empty")
+	}
+	f.data[key] = value
+	return nil
+}
+
+func (f *Frontmatter) Has(key string) bool {
+	_, exists := f.data[key]
+	return exists
+}
+
+func (f *Frontmatter) Delete(key string) {
+	delete(f.data, key)
+}
+
+func (f *Frontmatter) Keys() []string {
+	keys := make([]string, 0, len(f.data))
+	for k := range f.data {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
+func (f *Frontmatter) Data() map[string]interface{} {
+	result := make(map[string]interface{})
+	for k, v := range f.data {
+		result[k] = v
+	}
+	return result
+}
+
+func (f *Frontmatter) IsEmpty() bool {
+	return len(f.data) == 0
+}
+
+func (f *Frontmatter) Merge(other *Frontmatter) *Frontmatter {
+	merged := NewFrontmatter()
+
+	for k, v := range f.data {
+		merged.data[k] = v
+	}
+
+	for k, v := range other.data {
+		merged.data[k] = v
+	}
+
+	return merged
+}
+
+func (f *Frontmatter) Clone() *Frontmatter {
+	cloned := NewFrontmatter()
+	for k, v := range f.data {
+		cloned.data[k] = deepCopy(v)
+	}
+	return cloned
+}
+
+func deepCopy(src interface{}) interface{} {
+	if src == nil {
+		return nil
+	}
+
+	switch v := src.(type) {
+	case map[string]interface{}:
+		dst := make(map[string]interface{})
+		for key, val := range v {
+			dst[key] = deepCopy(val)
+		}
+		return dst
+	case []interface{}:
+		dst := make([]interface{}, len(v))
+		for i, val := range v {
+			dst[i] = deepCopy(val)
+		}
+		return dst
+	default:
+		return v
+	}
+}