Add tool add_sub_issue

[elsewhat]

Describe the feature or problem you’d like to solve

Sub-issues are a popular feature used in extensively in many enterprise projects.
Currently there is no method of adding a subissue.

Support for add_sub_issue is sufficient as the issue could be created with create_issue
(MCP clients and their LLM will typically be able to chain the tools together)

Proposed solution

• Add tool add_sub_issue
• Ensure get_issue lists the sub-issues with their title and if their open/closed.

Additional context

GraphQL - AddSubIssue mutation
REST API - Add Sub-issue

[aryasoni98]

Hi @elsewhat,
I would like to work on this issue. Let me know if it's available or if there are any specific guidelines to follow.

[SamMorrowDrums]

Closing as dupe of #154