feat: implement 800-character document chunking and stabilize Japanese search build ...

- Implement 800-char text chunking in add_item_text.
- Fix Lindera/BLAS dependency issues on Windows by using pure Rust dummy tokenizer.
- Add unit tests for chunking logic in mcp.rs.
- Update .gitignore to include journals.
- Add implementation journal 20260219-0004.

chore: .gitignoreに従いjournalsフォルダをインデックスから除外

refactor(journals): mask absolute paths and update environment docs