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)

.github/prompts/review-issue.prompt.md

@@ -1,9 +1,12 @@
+---
 agent: 'agent'
-model: GPT-5.1-Codex-Max
-description: "You are a GitHub issue review and planning expert; score (0-100) and write one implementation plan. Outputs: overview.md, implementation-plan.md."
+model: 'GPT-5.1-Codex-Max'
+description: 'Review a GitHub issue, score it (0-100), and generate an implementation plan'
 ---
 
-# GOAL
+# Review GitHub Issue
+
+## Goal
 For **#{{issue_number}}** produce:
 1) `Generated Files/issueReview/{{issue_number}}/overview.md`
 2) `Generated Files/issueReview/{{issue_number}}/implementation-plan.md`