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/note.go

@@ -0,0 +1,190 @@
+package domain
+
+import (
+	"path/filepath"
+	"regexp"
+	"strings"
+	"time"
+)
+
+type Note struct {
+	path         string
+	content      string
+	frontmatter  *Frontmatter
+	outlinks     []*WikiLink
+	tags         []*Tag
+	createdAt    time.Time
+	modifiedAt   time.Time
+}
+
+func NewNote(path, content string) (*Note, error) {
+	if err := validatePath(path); err != nil {
+		return nil, err
+	}
+
+	note := &Note{
+		path:        path,
+		content:     content,
+		frontmatter: NewFrontmatter(),
+		outlinks:    make([]*WikiLink, 0),
+		tags:        make([]*Tag, 0),
+		createdAt:   time.Now(),
+		modifiedAt:  time.Now(),
+	}
+
+	if err := note.extractLinks(); err != nil {
+		return nil, err
+	}
+
+	if err := note.extractTags(); err != nil {
+		return nil, err
+	}
+
+	return note, nil
+}
+
+func (n *Note) Path() string {
+	return n.path
+}
+
+func (n *Note) Content() string {
+	return n.content
+}
+
+func (n *Note) Frontmatter() *Frontmatter {
+	return n.frontmatter
+}
+
+func (n *Note) Outlinks() []*WikiLink {
+	return n.outlinks
+}
+
+func (n *Note) Tags() []*Tag {
+	return n.tags
+}
+
+func (n *Note) CreatedAt() time.Time {
+	return n.createdAt
+}
+
+func (n *Note) ModifiedAt() time.Time {
+	return n.modifiedAt
+}
+
+func (n *Note) UpdateContent(content string) error {
+	n.content = content
+	n.modifiedAt = time.Now()
+
+	n.outlinks = make([]*WikiLink, 0)
+	n.tags = make([]*Tag, 0)
+
+	if err := n.extractLinks(); err != nil {
+		return err
+	}
+
+	return n.extractTags()
+}
+
+func (n *Note) SetFrontmatter(fm *Frontmatter) {
+	n.frontmatter = fm
+	n.modifiedAt = time.Now()
+}
+
+func (n *Note) AddTag(tag *Tag) error {
+	for _, existingTag := range n.tags {
+		if existingTag.Name() == tag.Name() {
+			return nil
+		}
+	}
+
+	n.tags = append(n.tags, tag)
+	n.modifiedAt = time.Now()
+	return nil
+}
+
+func (n *Note) RemoveTag(tagName string) error {
+	for i, tag := range n.tags {
+		if tag.Name() == tagName {
+			n.tags = append(n.tags[:i], n.tags[i+1:]...)
+			n.modifiedAt = time.Now()
+			return nil
+		}
+	}
+	return nil
+}
+
+func (n *Note) HasTag(tagName string) bool {
+	for _, tag := range n.tags {
+		if tag.Name() == tagName {
+			return true
+		}
+	}
+	return false
+}
+
+func (n *Note) ExtractLinks() []*WikiLink {
+	return n.outlinks
+}
+
+func (n *Note) WordCount() int {
+	words := strings.Fields(n.content)
+	return len(words)
+}
+
+func (n *Note) CharacterCount() int {
+	return len(n.content)
+}
+
+func (n *Note) extractLinks() error {
+	linkRegex := regexp.MustCompile(`\[\[([^\]]+)\]\]`)
+	matches := linkRegex.FindAllStringSubmatch(n.content, -1)
+
+	for _, match := range matches {
+		if len(match) >= 2 {
+			link, err := ParseWikiLink(match[1])
+			if err != nil {
+				continue
+			}
+			n.outlinks = append(n.outlinks, link)
+		}
+	}
+
+	return nil
+}
+
+func (n *Note) extractTags() error {
+	tagRegex := regexp.MustCompile(`#([a-zA-Z0-9_/]+)`)
+	matches := tagRegex.FindAllStringSubmatch(n.content, -1)
+
+	for _, match := range matches {
+		if len(match) >= 2 {
+			tag, err := NewTag(match[1])
+			if err != nil {
+				continue
+			}
+			n.AddTag(tag)
+		}
+	}
+
+	return nil
+}
+
+func validatePath(path string) error {
+	if path == "" {
+		return ErrInvalidPath
+	}
+
+	if !strings.HasSuffix(path, ".md") {
+		return ErrInvalidPath
+	}
+
+	if strings.Contains(path, "..") {
+		return ErrInvalidPath
+	}
+
+	if !filepath.IsAbs(path) && strings.HasPrefix(path, "/") {
+		return ErrInvalidPath
+	}
+
+	return nil
+}