Add Copilot-Instructions label policy rule (#6440)

## 📖 Description

Adds a policy-bot rule to automatically apply a `Copilot-Instructions`
label to pull requests that modify `.github/copilot-instructions.md` or
any `.github/instructions/*.instructions.md` file, so reviewers get
extra visibility when Copilot's guidance changes.

The rule triggers on PR Opened and Synchronize, matches files via regex,
and is guarded with a
ot: hasLabel check so it won't re-fire once the label is already
applied.

Created with GitHub Copilot's assistance.

## 🔗 References

Closes #6439

## 🔍 Validation

- Validated the YAML with PowerShell's powershell-yaml module
(ConvertFrom-Yaml).
- Cross-checked all conditions/actions (payloadType, isAction,
ilesMatchPattern, hasLabel, �ddLabel) against the documented
GitOps.PullRequestIssueManagement policy schema.
- Mirrors the existing idempotency pattern already used elsewhere in
this file.

## ✅ Checklist

- [x] Signed the [Contributor License
Agreement](https://cla.opensource.microsoft.com)
- [x] Linked to an issue
- [ ] Updated [Release Notes](../doc/ReleaseNotes.md) (if applicable)
- [ ] Updated documentation (if applicable)
- [ ] Updated [Copilot instructions](.github/copilot-instructions.md)
(if build, architecture, or conventions changed)

## 📋 Issue Type

- [ ] Bug fix
- [ ] Feature
- [x] Task

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/winget-cli/pull/6440)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: denelon

.github/policies/labelManagement.issueOpened.yml

@@ -15,6 +15,23 @@ configuration:
               action: Opened
         then:
           - addCodeFlowLink
+      - description: Add Copilot-Instructions label to PRs modifying Copilot instruction files
+        if:
+          - payloadType: Pull_Request
+          - or:
+              - isAction:
+                  action: Opened
+              - isAction:
+                  action: Synchronize
+          - filesMatchPattern:
+              pattern: ^\.github/(copilot-instructions\.md|instructions/.+\.instructions\.md)$
+              matchAny: true
+          - not:
+              hasLabel:
+                label: Copilot-Instructions
+        then:
+          - addLabel:
+              label: Copilot-Instructions
       - description: Check for modifications of Localization files in new PRs
         if:
           - payloadType: Pull_Request