feat: add inline comment support for merge requests

[kishan0725]

Add support for creating inline/line-level comments on merge request diffs using GitLab's Discussions API. This enables AI assistants to provide precise, line-specific code review feedback similar to human reviewers.

What's Added

New Tools

• gitlab_create_merge_request_discussion: Full-featured tool for creating
  inline comments with manual commit SHA specification
• gitlab_create_merge_request_discussion_simple: Simplified tool that
  automatically fetches commit SHAs from the merge request

Implementation Details

• Created src/types/repository.types.ts with proper TypeScript interfaces
  for discussion positions and diff references
• Added handlers in src/handlers/repository-handlers.ts using GitLab's
  Discussions API endpoint
• Registered tools in tool registry
• Auto-generated documentation in TOOLS.md

Usage Example

The simplified version makes it easy to add inline comments:

{
  "project_id": "my-project",
  "merge_request_iid": 42,
  "body": "Consider using async/await here for better readability",
  "file_path": "src/server.ts",
  "line_number": 45,
  "line_type": "new"
}

[kishan0725]

Closing the PR as updated PR is #6