docs: replace copilot instructions with AGENTS.md (#794)

## Summary
- replace the root `AGENTS.md` symlink with a real file
- move the agent instructions out of `.github/copilot-instructions.md`
into `AGENTS.md`
- trim the content down to concise, repo-specific guidance and add a
short repository orientation

## Testing
- not run (docs-only change)

Author: eifinger

.github/copilot-instructions.md

@@ -0,0 +1,13 @@
+# setup-uv agent notes
+
+This repository is a TypeScript-based GitHub Action for installing `uv` in GitHub Actions workflows. It also supports restoring/saving the `uv` cache and optional managed-Python caching.
+
+- The published action runs the committed bundles in `dist/`, not the TypeScript in `src/`. After any code change, run `npm run package` and commit the resulting `dist/` updates.
+- Standard local validation is:
+  1. `npm ci --ignore-scripts`
+  2. `npm run all`
+- `npm run check` uses Biome (not ESLint/Prettier) and rewrites files in place.
+- User-facing changes are usually multi-file changes. If you add or change inputs, outputs, or behavior, update `action.yml`, the implementation in `src/`, tests in `__tests__/`, relevant docs/README, and then re-package.
+- The easiest areas to regress are version resolution and caching. When touching them, add or update tests for precedence, cache invalidation, and cross-platform path behavior.
+- Workflow edits have extra CI-only checks (`actionlint` and `zizmor`); `npm run all` does not cover them.
+- Before finishing, make sure validation does not leave generated or formatting-only diffs behind.