Add create_discussion tool to discussions toolset

[furkankoykiran]

Closes #1517

This PR adds a new create_discussion tool to the discussions toolset, enabling programmatic creation of GitHub Discussions through the MCP server.

Changes

• Added CreateDiscussion function in pkg/github/discussions.go using GraphQL mutation
• Implemented comprehensive unit tests covering success and error scenarios
• Updated discussions toolset to include the new write tool
• Generated toolsnap for schema validation

Testing

• All existing tests pass
• New tests cover:
  • Successful discussion creation
  • Organization-level discussions
  • Missing parameter validation
  • Repository and category not found errors
• Linter passes with no issues
• Documentation generated via script/generate-docs

The tool follows existing patterns in the codebase and integrates cleanly with the current discussions toolset structure.