Add labeler policy to auto-apply int:azure-specs label on linter rule changes (#4517)

## Summary

Adds a labeling policy (not a workflow) that automatically applies the
`int:azure-specs` label to PRs when linter rule source files are
modified.

## Approach

Uses the repo's existing `.github/policies/` mechanism (same as
`prs.triage.generated.yml`) rather than a GitHub Actions workflow. The
policy triggers on `Pull_Request` events and applies the label when
files match:

- `packages/typespec-azure-core/src/rules/**/*`
- `packages/typespec-azure-resource-manager/src/rules/**/*`
- `packages/typespec-client-generator-core/src/rules/**/*`

## Why

The `int:azure-specs` label triggers the External Integration workflow
which validates that linter rule changes don't break existing Azure
service specs. This automation ensures it's never forgotten.

Closes #4516

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: markcowl <1054056+markcowl@users.noreply.github.com>
Co-authored-by: Mark Cowlishaw <markcowl@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.github/policies/prs.linter-rule-integration.yml

@@ -0,0 +1,22 @@
+id: prs.linter-rule-integration
+name: Auto-apply int:azure-specs label on linter rule changes
+description: >-
+  Automatically applies the int:azure-specs label to PRs that modify linter rule
+  source files, ensuring the External Integration check is always triggered.
+resource: repository
+disabled: false
+configuration:
+  resourceManagementConfiguration:
+    eventResponderTasks:
+      - if:
+          - payloadType: Pull_Request
+        then:
+          - if:
+              - includesModifiedFiles:
+                  files:
+                    - packages/typespec-azure-core/src/rules/**/*
+                    - packages/typespec-azure-resource-manager/src/rules/**/*
+                    - packages/typespec-client-generator-core/src/rules/**/*
+            then:
+              - addLabel:
+                  label: int:azure-specs