feat: Phase 2 Security Enhancement

- pkg/crypto: AES-256-GCM encryption with PBKDF2 key derivation
  - 100k iterations, 16-byte salt, SHA-256
  - Encrypt/Decrypt/IsEncrypted/HashPassword
- Storage layer encryption:
  - JSONStorage.SetPassword() enables transparent encrypt/decrypt
  - readJSON auto-decrypts, replace* auto-encrypts
- pkg/knownhosts: TOFU host key verification
  - Verify/Add/Remove host keys
  - HostKeyCallback for SSH config
- SSH client security:
  - SetHostKeyCallback() replaces InsecureIgnoreHostKey()
  - SetPassphraseCallback() for encrypted private keys
  - getKeySigner() tries passphrase on encrypted keys
- Models: AppConfig gains EncryptionEnabled, PasswordHash, KnownHostsFile

CHANGELOG.md

@@ -116,3 +116,35 @@
 - Initial MVP release, all core features functional
 - Encrypted storage planned for future release
 - Interactive shell in Go SSH direct mode not yet available
+
+## Phase 2 — Security Enhancement
+
+### AES-256-GCM Encryption
+- New `pkg/crypto/crypto.go`: AES-256-GCM encrypt/decrypt with PBKDF2 key derivation
+- 100,000 iterations, 16-byte salt, SHA-256 key derivation
+- `Encrypt(plaintext, password)` → base64(salt + nonce + ciphertext)
+- `Decrypt(encoded, password)` → plaintext
+- `IsEncrypted(data)` checks if data looks like encrypted content
+
+### Storage Layer Encryption
+- `JSONStorage` now has `password` field for master encryption key
+- `SetPassword()`, `GetPassword()`, `IsEncrypted()` methods
+- `readJSON()` auto-decrypts if password is set and data is encrypted
+- `replaceHosts/KeyPairs/Snippets()` auto-encrypt before writing
+- All existing CRUD operations transparently encrypt/decrypt
+
+### Known Hosts Verification
+- New `pkg/knownhosts/knownhosts.go`: TOFU (Trust-On-First-Use) model
+- `KnownHosts` manages `known_hosts` file (JSON format)
+- `Verify()` checks if host key matches stored key
+- `HostKeyCallback()` returns `cryptossh.HostKeyCallback` for SSH config
+- Warns on key mismatch (potential MITM attack)
+
+### SSH Client Security
+- `Client` now supports `hostKeyCallback` and `passphraseCallback`
+- `SetHostKeyCallback()` — replaces `InsecureIgnoreHostKey()`
+- `SetPassphraseCallback()` — prompts for passphrase on encrypted keys
+- `getKeySigner()` tries passphrase callback if key is encrypted
+
+### Models Updated
+- `AppConfig`: added `EncryptionEnabled`, `PasswordHash`, `KnownHostsFile`