refactor: Consolidate issue assignment workflows

[parvninama]

Summary:

This PR refactors the issue assignment automation into a reusable, configuration-driven workflow that supports multiple skill levels without duplicating logic.

Instead of maintaining separate assignment guards for each difficulty level, the assignment flow now resolves repository configuration and prerequisites dynamically from config.js, making it easier to extend and maintain.

Changes:

• Introduced a shared assignment engine (runAssignmentFlow) to handle all /assign requests.
• Moved repository labels, skill hierarchy, and prerequisite rules into configuration.
• Added generic prerequisite enforcement based on configured skill levels.
• Added one-time reminder comments for contributors who express interest without using /assign.
• Added reusable comment builders and spam-handling helpers.
• Centralized GitHub API interactions into shared helper modules.
• Prerequisite validation now happens before assigning an issue, preventing invalid assignments instead of removing assignees afterward.
• Assignment limits and spam restrictions are enforced as part of the /assign workflow.
• Manual assignments made by maintainers are intentionally not overridden. The workflow governs contributor-initiated /assign requests, while maintainer assignments are treated as trusted actions.

Tests:

• In progress

Linked Issue:

• Fixes Consolidate Issue Assignment Workflows #2371

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