feat(matrix): support encrypted messages with E2EE

- Add `crypto_database_path` and `crypto_passphrase` configuration
- Integrate cryptohelper for decrypting `m.room.encrypted` events
- Handle both plaintext and encrypted messages in `handleMessageEvent`
- Enable `goolm` build tag for libsignal crypto support

Fixes #1840.

pkg/config/config.go

@@ -603,6 +603,8 @@ type MatrixConfig struct {
 	Placeholder        PlaceholderConfig   `json:"placeholder,omitempty"`
 	ReasoningChannelID string              `json:"reasoning_channel_id"     env:"PICOCLAW_CHANNELS_MATRIX_REASONING_CHANNEL_ID"`
 	secDirty           bool
+	CryptoDatabasePath string `json:"crypto_database_path,omitempty" env:"PICOCLAW_CHANNELS_MATRIX_CRYPTO_DATABASE_PATH"`
+	CryptoPassphrase   string `json:"crypto_passphrase,omitempty" env:"PICOCLAW_CHANNELS_MATRIX_CRYPTO_PASSPHRASE"`
 }
 
 // AccessToken returns the Matrix access token