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/wikilink.go

@@ -0,0 +1,78 @@
+package domain
+
+import (
+	"fmt"
+	"strings"
+)
+
+type WikiLink struct {
+	target  string
+	alias   string
+	section string
+}
+
+func NewWikiLink(target string, alias string, section string) (*WikiLink, error) {
+	if target == "" {
+		return nil, fmt.Errorf("wikilink target cannot be empty")
+	}
+
+	return &WikiLink{
+		target:  target,
+		alias:   alias,
+		section: section,
+	}, nil
+}
+
+func (w *WikiLink) Target() string {
+	return w.target
+}
+
+func (w *WikiLink) Alias() string {
+	return w.alias
+}
+
+func (w *WikiLink) Section() string {
+	return w.section
+}
+
+func (w *WikiLink) IsValid() bool {
+	return w.target != ""
+}
+
+func (w *WikiLink) String() string {
+	var parts []string
+
+	if w.alias != "" {
+		return fmt.Sprintf("[[%s|%s]]", w.target, w.alias)
+	}
+
+	if w.section != "" {
+		return fmt.Sprintf("[[%s#%s]]", w.target, w.section)
+	}
+
+	return fmt.Sprintf("[[%s]]", w.target)
+}
+
+func ParseWikiLink(linkText string) (*WikiLink, error) {
+	linkText = strings.Trim(linkText, "[]")
+
+	if linkText == "" {
+		return nil, fmt.Errorf("empty wikilink")
+	}
+
+	var target, alias, section string
+
+	if strings.Contains(linkText, "|") {
+		parts := strings.SplitN(linkText, "|", 2)
+		target = strings.TrimSpace(parts[0])
+		alias = strings.TrimSpace(parts[1])
+	} else if strings.Contains(linkText, "#") {
+		parts := strings.SplitN(linkText, "#", 2)
+		target = strings.TrimSpace(parts[0])
+		section = strings.TrimSpace(parts[1])
+	} else {
+		target = strings.TrimSpace(linkText)
+	}
+
+	return NewWikiLink(target, alias, section)
+}