feat: protect sensitive files and auto-backup before modifications

Implements GitHub issues #104 and #102:

**#104: Sensitive File Protection**
- Block writes to .env, credentials, SSH keys, cloud configs
- Categories: environment, credentials, ssh_keys, api_tokens, certificates, cloud_config
- Warn on reads of .env files (may contain secrets)
- Block writes to id_rsa, *.pem, *.key, credentials.json, etc.

**#102: Auto-Backup System**
- Automatic backup before every write/edit operation
- Backups stored in .codetyper-backup/ (preserves directory structure)
- Max 10 backups per file
- 7-day retention with auto-cleanup
- listBackups, getLatestBackup, restoreFromBackup functions

Closes #104
Closes #102

src/constants/backup.ts

@@ -0,0 +1,42 @@
+/**
+ * Backup System Constants
+ *
+ * Configuration for the automatic file backup system.
+ */
+
+/**
+ * Default backup configuration
+ */
+export const BACKUP_DEFAULTS = {
+  /** Whether backup is enabled by default */
+  ENABLED: true,
+  /** Directory to store backups (relative to working dir) */
+  DIRECTORY: ".codetyper-backup",
+  /** Maximum number of backups to keep per file */
+  MAX_BACKUPS_PER_FILE: 10,
+  /** Number of days to retain backups */
+  RETENTION_DAYS: 7,
+  /** File extension for backups */
+  EXTENSION: ".bak",
+};
+
+/**
+ * Backup messages
+ */
+export const BACKUP_MESSAGES = {
+  CREATED: (backupPath: string) => `Backup created: ${backupPath}`,
+  RESTORED: (filePath: string) => `Restored from backup: ${filePath}`,
+  CLEANUP: (count: number) => `Cleaned up ${count} old backup(s)`,
+  NO_BACKUP: (filePath: string) => `No backup found for: ${filePath}`,
+  BACKUP_FAILED: (error: string) => `Backup failed: ${error}`,
+  LIST_HEADER: "Available backups:",
+  NO_BACKUPS: "No backups available",
+};
+
+/**
+ * Gitignore entry for backup directory
+ */
+export const BACKUP_GITIGNORE_ENTRY = `
+# CodeTyper backup files
+.codetyper-backup/
+`;