ADR 0019: Use pathspec for pinned gitignore matching

Date: 2026-01-14

Context: Repo-state embeddings rely on pinned repository-root ignore rules (mapelites_repo_state_ignore_text) and must support gitignore features such as negation and directory rules with correct semantics. Decision: Compile the pinned ignore text with pathspec.gitignore.GitIgnoreSpec and use match_file() to filter git-root-relative paths. Constraints: Ignore matching remains root-only (no nested .gitignore files and no global excludes). Consequences: Ignore behavior matches gitignore semantics more closely; a small number of edge cases may change eligibility and should be validated by tests when upgrading.