Add nested .gitignore support

### Summary
Evaluate `.gitignore` files inside subdirectories and merge their rules into the global pattern list.
This ensures ContextWeaver respects nested ignore files the same way Git and Repomix do.

### Acceptance Criteria
- Files under nested folders are skipped or kept based on their local `.gitignore`
- Unit tests verify nested pattern behavior

Start by locating the existing global ignore-pattern handling and its unit tests. Trace how files in subdirectories are evaluated, then add coverage for local .gitignore rules and verify that nested files are skipped or kept according to those rules.

Written by the indexing model from the issue text.