Docs: consolidate instructions and fix prompt frontmatter (#44610)

Title: Docs: consolidate Copilot instructions and prompt metadata

## Summary
- Consolidated AI guidance into a root AGENTS.md and new
`.github/instructions` files, removing older per-folder instructions.
- Scoped instruction files for pipelines, common libraries,
runner/settings UI, prompts, and simplified
`.github/copilot-instructions.md` to point to the sources of truth.
- Fixed prompt frontmatter (YAML markers, quoted fields, headings)
across built-in prompt files.
- Most instructions.md is from https://github.com/github/awesome-copilot

## Testing
- Not run (documentation/instructions-only change)