🔐 feat: Add token export/import for ephemeral environments

- Add token CLI subcommand with export/import operations
- Enable OAuth2 token persistence across clean environments
- Support for containers, CI/CD, and ephemeral compute workflows
- Compress tokens with gzip and encode as base64 for env vars
- Automatic token restoration from CULL_GMAIL_TOKEN_CACHE
- Secure file permissions (600) on restored tokens
- Add comprehensive error handling for token operations
- Update dependencies: base64, flate2, serde_json

This feature enables cull-gmail to run in ephemeral environments
like Docker containers and CI/CD pipelines without re-authentication
by exporting tokens once and restoring them via environment variables.

src/error.rs

@@ -51,4 +51,13 @@ pub enum Error {
     /// Invalid message age specification
     #[error("Invalid message age: {0}")]
     InvalidMessageAge(String),
+    /// Token not found or missing
+    #[error("Token error: {0}")]
+    TokenNotFound(String),
+    /// File I/O error with context
+    #[error("File I/O error: {0}")]
+    FileIo(String),
+    /// Serialization/deserialization error
+    #[error("Serialization error: {0}")]
+    SerializationError(String),
 }