Addresses#670

Overview
This PR expands the functionality of thelist_discussions tool. It adds

• TheupdatedAt andauthor.login (i.e. username) fields to the discussions payload in thelist_discussions tool
• The ability to order discussions by field (updated_at andcreated_at) and direction (ascending order, descending order)

Refactoring
The PR includes extensive refactoring of thelist_discussions tool to handle optional parameters in our graphql queries. This was needed because the githubv4 library requires GraphQL queries to be defined using Go struct tags that must be known at compile time, so we could not dynamically construct query strings at runtime based on user input.

The changes made include

• Introduced reusableDiscussionFragment struct to avoid field duplication
• Created 4 pre-defined query variants:
  • BasicNoOrder: No optional parameters (uses GitHub API's defaults)
  • BasicWithOrder: Only ordering parameters
  • WithCategoryNoOrder: Only category filtering
  • WithCategoryAndOrder: Both category and ordering
• Added runtime query selection logic to choose the appropriate variant based on user input

Tests
I updated the tests indiscussions_test.go to match the new queries, and to test the new ordering functionality with different sorting combinations.

Demo

Simple "list discussions" prompt

Ordering: from most recently created to least recently created

Ordering: from least recently updated to most recently updated

Less explicit prompt: show me discussions from last 10 days

Ordering with weaker model (GPT-4o)

Note: this PR adds author data to the payload of thelist_discussions tool but doesn't add support for filtering by author when listing discussions because that is not supported on the discussions field in the API, and it would need to be achieved in a separate search tool.