Add repository Copilot customizations for openbsw contributors

[simon-d-bmw]

Describe the feature
Add repository-level GitHub Copilot customizations for openbsw so AI-assisted work follows the project’s structure, build/test workflow, and contribution expectations more consistently.

Proposed Solution
Describe your preferred solution, addressing:

• Add a repository instruction file such as .github/copilot-instructions.md with openbsw-specific guidance for architecture discovery, CMake preset usage, focused testing, minimal diffs, and avoiding unrelated refactors
• Add a small set of reusable skills for recurring tasks, for example:
  • fixing clang-tidy findings in production code
  • navigating the module structure and locating the right library/executable/test target
  • implementing or reviewing changes while preserving existing style and public APIs
• Add one or more repository agents focused on common openbsw workflows, for example:
  • exploring the codebase and identifying the right module or target
  • handling static-analysis remediation with project-specific constraints
• Keep the public customizations scoped to information that belongs in the open source repository: build/test commands, directory structure, contribution expectations, and public coding conventions
• Avoid embedding company-internal processes, private remotes, or internal ticketing/review flows in the upstream customization files
• Consider adding lightweight validation or example prompts so the customizations remain maintainable as the repository evolves
• Use cases:
  • helping new contributors navigate a large multi-module codebase
  • making AI-assisted changes more consistent and reviewable
  • reducing repeated prompt setup for common maintenance tasks
• This request is not specific to one operating system, compiler, or toolchain; the guidance should stay portable and repository-focused
• Potential limitations / future considerations:
  • the customizations should stay concise so they do not over-constrain contributors
  • the files should be generic enough to remain useful across supported openbsw environments
  • repository guidance may need periodic updates as build/test workflows change

Additional context
The repository already has a strong .github layout for templates and workflows, so this seems like a natural extension. Repository-level Copilot customizations would help contributors use AI tools more consistently without relying on private or local-only setup.

[rolandreichweinbmw]

OTOH, if this is not Github or Copilot specific, it could be doc/ai-instructions.md (or similar) as well?

Despite some already existing Github specific parts, we try to limit vendor lock-in.

Of course, at the point where some of this contains contents specific to Github or Copilot, it is reasonable to locate it accordingly.