Add automated PR comment for untested linter rule changes

[Copilot]

When linter rules are modified without specifying rules to test (via labels or PR description), the Staging Lint Checks workflow now posts a reminder comment directing developers to validate their changes.

Changes

• Workflow: Added pull-requests: write permission and passed github API client to script
• Change detection: Detects modifications to packages/rulesets/src/{spectral,native}/functions/** using startsWith() matching to avoid false positives
• Comment logic: Posts reminder only when:
  1. Linter rule files changed
  2. No rules specified via test-* labels or rules: in PR body
  3. No existing comment (uses HTML marker <!-- staging-lint-checks-reminder --> for deduplication)
• Testing: Added 16 unit tests covering path detection, duplicate prevention, and error handling

Comment posted

⚠️ **Linter Rule Changes Detected**

This PR modifies linter rule files, but no specific rules were selected for testing.

Make sure to validate the changes to the linter rules using 'Staging Lint Checks' workflow.

To test your changes, add one of the following:
- A label in the format `test-<RuleName>` (e.g., `test-PostResponseCodes`)
- A line in the PR description: `rules: RuleName1, RuleName2`

The workflow triggers on PR open/sync/edit/label events, so duplicate prevention ensures clean comment threads.

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