docs: document src/services/types.rs, src/modes/common.rs, and src/services/error.rs

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

PLAN.md

@@ -233,16 +233,16 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
 40. **src/filter_parser.rs** (duplicate of filter_parser.rs)
     - Same issues as above.
 
-41. **src/services/types.rs**
+41. **src/services/types.rs** [DONE]
     - `ItemWithMeta` and `ItemWithContent` structs: Partial.
     - `meta_as_map()` method: No doc.
 
-42. **src/modes/common.rs**
+42. **src/modes/common.rs** [DONE]
     - `OutputFormat` enum: No doc.
     - `ColumnType` enum: Partial.
     - Functions (`get_meta_from_env`, `format_size`, etc.): Partial.
 
-43. **src/services/error.rs**
+43. **src/services/error.rs** [DONE]
     - `CoreError` enum: Partial variants.
 
 44. **src/services/compression_service.rs**