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/errors.go

@@ -0,0 +1,12 @@
+package domain
+
+import "errors"
+
+var (
+	ErrNoteNotFound      = errors.New("note not found")
+	ErrNoteAlreadyExists = errors.New("note already exists")
+	ErrInvalidPath       = errors.New("invalid path")
+	ErrEmptyContent      = errors.New("content cannot be empty")
+	ErrInvalidTag        = errors.New("invalid tag format")
+	ErrCircularLink      = errors.New("circular link detected")
+)