Add BUG-FIXER custom Copilot agent

[ivanvpetrov]

Summary

Adds a custom Copilot agent (BUG-FIXER) for fixing bugs in the IgniteUI Angular component library.

What's included

• Defines the agent's role as a senior Angular/TypeScript developer focused on minimal, safe bug fixes
• Structured TDD-first workflow:
  1. Understand the issue and identify root cause
  2. Write a failing test first to reproduce the bug
  3. Implement the smallest fix to make it pass
  4. Verify no regressions
• References key repository resources: CONTRIBUTING.md, coding guidelines, and domain skills for components, grids, and theming
• Documents the Jasmine/Karma test setup and all area-specific test scripts (test:lib:grid, test:lib:others, etc.)
• Prompts the agent to include migration schematics for any breaking changes

TEST Bug Fixing jobs:

PR #17048

[Copilot]

Pull request overview

Adds a repository-scoped custom Copilot agent configuration aimed at guiding safe, minimal bug fixes in the Ignite UI for Angular codebase, with an explicit TDD-first workflow and pointers to key repo resources.

Changes:

• Introduces a BUG-FIXER custom agent definition under .github/agents/
• Documents a structured bug-fixing workflow emphasizing failing-test-first (Jasmine/Karma) and minimal fixes
• Links to repo contribution guidelines, Copilot instructions, and domain skills references

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