feat: Initialize hosts-go project with foundational structure and core functionality

- Created activeContext.md and productContext.md to outline project goals and current focus.
- Established progress.md to track project milestones and tasks.
- Developed projectbrief.md detailing application overview, requirements, and directory structure.
- Documented systemPatterns.md to describe architecture and design patterns used.
- Compiled techContext.md to specify technologies and development setup.
- Implemented comprehensive unit tests in models_test.go for HostEntry and HostsFile functionalities.

memory-bank/systemPatterns.md

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+# System Patterns: hosts-go
+
+## Architecture Overview
+
+The hosts-go application follows a **layered architecture** with clear separation of concerns:
+
+```
+┌─────────────────────┐
+│     TUI Layer       │  ← Bubble Tea models, views, styles
+├─────────────────────┤
+│   Business Logic    │  ← Core domain logic, validation
+├─────────────────────┤
+│   System Interface  │  ← File I/O, DNS, permissions
+└─────────────────────┘
+```
+
+## Key Technical Decisions
+
+### 1. **Bubble Tea Framework**
+- **Why**: Provides mature TUI framework with event-driven architecture
+- **Pattern**: Model-View-Update (MVU) pattern for state management
+- **Components**: Main model orchestrates sub-components (list, detail, modal)
+
+### 2. **Internal Package Structure**
+```
+internal/
+├── tui/      # UI layer - Bubble Tea components
+├── core/     # Business logic - domain models and operations  
+└── utils/    # Shared utilities - helpers and common functions
+```
+
+### 3. **Permission Model**
+- **View mode default**: Read-only access, no sudo required
+- **Edit mode explicit**: User must explicitly enter edit mode
+- **Sudo on demand**: Only request elevated permissions when entering edit mode
+- **Graceful fallback**: Handle permission denied gracefully
+
+## Design Patterns in Use
+
+### 1. **Model-View-Update (MVU)**
+- **Model**: Application state (entries, selection, mode)
+- **View**: Rendering logic (two-pane layout, styles)
+- **Update**: State transitions based on user input
+
+### 2. **Command Pattern**
+- **Edit operations**: Encapsulate modifications as commands
+- **Undo support**: Commands can be reversed
+- **Validation**: Commands validate before execution
+
+### 3. **Repository Pattern**
+- **HostsRepository**: Abstract file operations
+- **MockRepository**: In-memory implementation for testing
+- **FileRepository**: Actual `/etc/hosts` file operations
+
+### 4. **Observer Pattern**
+- **DNS resolution**: Background updates to IP addresses
+- **File watching**: Detect external changes to hosts file
+- **Status updates**: Notify UI of operation progress
+
+## Component Relationships
+
+### TUI Components
+```
+MainModel
+├── ListModel (left pane)
+│   ├── Entry selection
+│   ├── Sorting controls
+│   └── Status indicators
+├── DetailModel (right pane)
+│   ├── Entry editing
+│   ├── Form validation
+│   └── DNS resolution
+└── ModalModel (overlays)
+    ├── Configuration
+    ├── Confirmations
+    └── Error dialogs
+```
+
+### Core Business Logic
+```
+Manager
+├── HostsParser (read/write hosts file)
+├── DNSResolver (hostname to IP resolution)
+├── Validator (entry validation)
+└── ConfigManager (user preferences)
+```
+
+## Critical Implementation Paths
+
+### 1. **File Operations**
+```go
+// Atomic file updates with backup
+1. Read current /etc/hosts → backup
+2. Parse entries → validate changes
+3. Write to temporary file → verify
+4. Atomic move temp → /etc/hosts
+5. Remove backup on success
+```
+
+### 2. **State Management**
+```go
+// Bubble Tea update cycle
+1. User input → Command
+2. Command → State change
+3. State change → View update
+4. View update → Screen render
+```
+
+### 3. **DNS Resolution**
+```go
+// Background IP resolution
+1. Extract hostnames from entries
+2. Resolve in background goroutines  
+3. Compare resolved vs current IPs
+4. Update UI with resolution status
+5. User chooses whether to update
+```
+
+### 4. **Edit Mode Transition**
+```go
+// Permission elevation
+1. User requests edit mode
+2. Check current permissions
+3. Request sudo if needed
+4. Validate file write access
+5. Enable editing controls
+```
+
+## Error Handling Strategy
+
+### 1. **Graceful Degradation**
+- **No sudo**: Continue in view-only mode
+- **File locked**: Show warning, allow retry
+- **DNS failure**: Show cached/manual IP options
+
+### 2. **Validation Layers**
+- **Input validation**: Real-time feedback on forms
+- **Business rules**: Validate complete entries
+- **System constraints**: Check file permissions, IP formats
+
+### 3. **Recovery Mechanisms**
+- **Backup restoration**: Automatic rollback on write failures
+- **State recovery**: Restore UI state after errors
+- **User guidance**: Clear error messages with suggested actions
+
+## Testing Architecture
+
+### 1. **Unit Tests**
+- **Pure functions**: Parser, validator, DNS resolver
+- **Mocked dependencies**: File system, network calls
+- **Edge cases**: Malformed files, network errors
+
+### 2. **Integration Tests**
+- **TUI workflows**: Complete user interactions
+- **File operations**: Real file system operations (in temp dirs)
+- **Permission scenarios**: Test sudo/non-sudo paths
+
+### 3. **Test Patterns**
+- **Table-driven tests**: Multiple input scenarios
+- **Mock interfaces**: Controllable external dependencies
+- **Golden files**: Expected output comparisons