docs: add project rules and module documentation to DESIGN.md

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

DESIGN.md

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+# PROJECT RULES - KEEP FIRST
+
+## Code - Modules
+
+### Main Module
+- `main.rs` - Entry point, CLI argument parsing, mode dispatching
+- Interacts with all mode modules based on user input
+- Handles database connection setup and data directory management
+
+### Mode Modules
+- `modes/save.rs` - Save new items with tags/metadata
+- `modes/get.rs` - Retrieve items by ID/tags
+- `modes/list.rs` - List items with filtering and formatting
+- `modes/delete.rs` - Delete items by ID
+- `modes/update.rs` - Update item tags/metadata
+- `modes/info.rs` - Show detailed item information
+- `modes/diff.rs` - Compare two items
+- `modes/status.rs` - Show system status and capabilities
+- `modes/common.rs` - Shared utilities for all modes
+
+### Database Module
+- `db.rs` - SQLite database operations
+- Handles items, tags, and metadata storage
+- Provides query functions for all modes
+- Manages database migrations
+
+### Compression Engine Module
+- `compression_engine.rs` - Trait and type definitions
+- `compression_engine/gzip.rs` - GZip implementation
+- `compression_engine/lz4.rs` - LZ4 implementation
+- `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
+
+### Meta Plugin Module
+- `meta_plugin.rs` - Trait and type definitions
+- `meta_plugin/program.rs` - External program wrapper
+- `meta_plugin/digest.rs` - Internal digest implementations
+
+### Plugins Module
+- `plugins.rs` - Shared plugin utilities
+- Contains `ProgramWriter` for external process communication
+
+## Standard Rules
+
+1. ALWAYS keep DESIGN.md updated with any architectural or design changes
+2. Follow Rust naming conventions and idioms
+3. Use anyhow for error handling throughout the codebase
+4. Maintain comprehensive logging with the log crate
+5. Write unit tests for critical functionality
+6. Document public APIs with rustdoc comments
+7. Keep modules focused on single responsibilities
+8. Prefer composition over inheritance
+9. Handle errors gracefully and provide meaningful error messages
+10. Ensure code is safe and avoids unsafe blocks where possible