refactor(chunk): Chunk 생성부의 aliases 리터럴 + store 컬럼 제거

kebab-chunk/* AST·md·tier2·pdf chunker 의 aliases: None 리터럴 삭제,
store-sqlite documents.rs chunks INSERT 컬럼/바인딩 + get_chunk 매핑에서
aliases 제거.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/kebab-chunk/src/md_heading_v1.rs

@@ -339,7 +339,6 @@ fn build_chunk(
         token_estimate,
         chunker_version: chunker_version.clone(),
         policy_hash: policy_hash.to_string(),
-        aliases: None,
     }
 }