refactor: Consolidate GitHub Workflow tools per Issue #8318

Optimizes MCP tool count by consolidating assign, label, and comment operations into 'manage_*' tools.

- Consolidates assign/unassign into 'manage_issue_assignees'
- Consolidates add/remove labels into 'manage_issue_labels'
- Consolidates create/update comments into 'manage_issue_comment'
- Updates API definition in openapi.yaml with detailed schemas
- Refactors IssueService to handle unified operations
- Removes redundant code from PullRequestService

Author: tobiu