Adds functionality to fetch details of a specific GitHub issue by number.
This includes:
- New GetIssueSchema for input validation
- Implementation of getIssue function using GitHub API
- Addition of get_issue tool to available tools list
- Handler for get_issue in CallToolRequestSchema
This allows users to retrieve complete issue information including:
- Issue metadata (title, body, state)
- Associated data (labels, assignees, milestone)
- Timestamps (created, updated, closed)
- Implemented `listIssues`, `updateIssue`, and `addIssueComment` functions to manage GitHub issues.
- Introduced corresponding schemas: `ListIssuesOptionsSchema`, `UpdateIssueOptionsSchema`, and `IssueCommentSchema`.
- Updated server request handlers to support new functionalities.
- Enhanced README with documentation for new features.
