docs(copilot): add pre-commit and common/common-test sync requirements to Copilot instructions (#44987)

This updates repository Copilot guidance to reflect two workflow
requirements from the issue: include `pre-commit` in expected checks,
and require test updates in `common-test` when `common` changes.

- **Copilot instruction updates**
- Added explicit instruction to run `pre-commit` before finalizing
changes.
- Added explicit instruction to keep Helm unit tests in
`charts/library/common-test` aligned with edits in
`charts/library/common`.

- **Scope**
- Changes are limited to `.github/copilot-instructions.md`
(documentation/config guidance only).

```md
- Run `pre-commit` on your changes before finalizing.
- When editing `charts/library/common`, also update the Helm unit tests in `charts/library/common-test` accordingly.
```

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.github/copilot-instructions.md

@@ -0,0 +1,8 @@
+# Copilot instructions for TrueCharts
+
+- This repository primarily contains Helm charts under `charts/`.
+- When changing chart files (other than docs like `README.md`), also bump that chart version in its `Chart.yaml`.
+- Keep changes minimal and scoped to the requested chart or library component.
+- Run `pre-commit` on your changes before finalizing.
+- When editing `charts/library/common`, also update the Helm unit tests in `charts/library/common-test` accordingly.
+- For common library changes, run `./run_common_tests.sh` (requires the Helm `unittest` plugin).