refactor: integrate digest functionality into meta plugins and remove digest_engine module

Co-authored-by: aider (openai/andrew.openrouter.qwen.qwen3-coder) <aider@aider.chat>

DESIGN.md

@@ -4,15 +4,16 @@
 
 1. ALWAYS keep DESIGN.md updated with any architectural or design changes
 2. ALWAYS keep project rules first in this document
-3. Follow Rust naming conventions and idioms
-4. Use anyhow for error handling throughout the codebase
-5. Maintain comprehensive logging with the log crate
-6. Write unit tests for critical functionality
-7. Document public APIs with rustdoc comments
-8. Keep modules focused on single responsibilities
-9. Prefer composition over inheritance
-10. Handle errors gracefully and provide meaningful error messages
-11. Ensure code is safe and avoids unsafe blocks where possible
+3. ALWAYS use git commands to remove or move files (`git rm`, `git mv`, etc.)
+4. Follow Rust naming conventions and idioms
+5. Use anyhow for error handling throughout the codebase
+6. Maintain comprehensive logging with the log crate
+7. Write unit tests for critical functionality
+8. Document public APIs with rustdoc comments
+9. Keep modules focused on single responsibilities
+10. Prefer composition over inheritance
+11. Handle errors gracefully and provide meaningful error messages
+12. Ensure code is safe and avoids unsafe blocks where possible
 
 ## Code - Modules
 
@@ -45,11 +46,10 @@
 - `compression_engine/none.rs` - No compression implementation
 - `compression_engine/program.rs` - External program wrapper
 
-### Digest Engine Module
-- `digest_engine.rs` - Trait and type definitions
-- `digest_engine/sha2.rs` - SHA-256 implementation
-- `digest_engine/none.rs` - No digest implementation
-- `digest_engine/program.rs` - External program wrapper
+### Digest Functionality
+- Digest functionality is now integrated into meta plugins
+- SHA-256 and other digest algorithms are implemented as meta plugins
+- External digest programs are supported through meta plugin program wrapper
 
 ### Meta Plugin Module
 - `meta_plugin.rs` - Trait and type definitions