Add writer tool

[ohdearquant]

Pull Request Description

Overview
This pull request introduces a new WriterTool, expanding LionAGI’s ability to handle file writing and saving. The WriterTool enforces a configurable root directory for all disk operations, preventing undesired file access outside the allowed path. It supports creating, opening, writing, and closing documents in memory, as well as saving text or chunk data to disk.

Key Changes

• Added WriterTool class and associated Pydantic models (WriterRequest, WriterResponse) for in-memory writing.
• Restricts writing to an allowed_root directory, ensuring safe I/O operations.
• Updated CI configurations to remove Python 3.13 from the matrix.
• Enhanced and reorganized documentation under docs/modules/tools.rst.
• Bumped version from 0.8.6 to 0.8.7 to reflect the new tool.

Why This PR?

• Improves file management by enabling partial or appended writes, chunk saving, and directory restriction.
• Complements existing ReaderTool, CoderTool, and GitHubTool for a broader range of local and sandboxed file manipulations.

How to Test

1. Initialize the WriterTool with an allowed_root path.
2. Call open to create or open a document.
3. Use write to append or overwrite content.
4. Verify you can list_docs, close docs, and save_file/save_chunks within the allowed directory.
5. Ensure writing outside the allowed path returns an error.

Issue Reference

• Closes Add writer tool #542 (Add writer tool)