Add CreateSubIssue function to combine issue creation and sub-issue linking

[Copilot]

This PR implements a new create_sub_issue tool that streamlines the workflow for creating issues that need to be linked as sub-issues to a parent issue.

What was added

New CreateSubIssue function

• Location: pkg/github/issues.go
• Tool name: create_sub_issue
• Functionality: Creates a new issue and automatically adds it as a sub-issue to a specified parent issue in a single operation

Parameters

The function accepts all standard issue creation parameters plus a required parent_issue_number:

• owner (required): Repository owner
• repo (required): Repository name
• parent_issue_number (required): The number of the parent issue
• title (required): Issue title
• body (optional): Issue body content
• assignees (optional): Usernames to assign to this issue
• labels (optional): Labels to apply to this issue
• milestone (optional): Milestone number
• type (optional): Type of this issue

Implementation details

The function performs two sequential operations:

1. Creates the issue using the GitHub Issues API
2. Automatically adds the created issue as a sub-issue to the parent using the SubIssue API

Error handling provides clear distinction between issue creation failures and sub-issue addition failures, helping users understand which step failed.

Testing

Added comprehensive test coverage with 5 scenarios:

• Successful creation with all parameters
• Successful creation with minimal parameters
• Issue creation failure handling
• Sub-issue addition failure handling (after successful issue creation)
• Parameter validation

Documentation

Updated README.md to include the new tool with complete parameter documentation.

Benefits

• Streamlined workflow: Single operation instead of requiring separate create_issue + add_sub_issue calls
• Consistent interface: Matches create_issue parameters exactly, just adds parent_issue_number
• Robust error handling: Clear error messages for both creation steps
• Full compatibility: Works with all existing issue creation features (labels, assignees, milestones, etc.)

Fixes #2.

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[JaclynCodes]

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[JaclynCodes]

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