feat: add reusable resolve-outdated-bot-comments workflow

[rnegron]

Summary

• Add reusable workflow for auto-resolving outdated bot review comments on PRs
• Triggered via workflow_call with configurable bots list input and GitHub App secrets
• Extracted from PostHog/posthog's existing workflow so other repos can adopt it

How to use

jobs:
  resolve:
    uses: PostHog/.github/.github/workflows/resolve-outdated-bot-comments.yml@main
    if: ${{ github.event.pull_request.head.repo.full_name == github.repository }}
    secrets:
      app_id: ${{ secrets.GH_APP_POSTHOG_REVIEW_COMMENT_RESOLVER_APP_ID }}
      private_key: ${{ secrets.GH_APP_POSTHOG_REVIEW_COMMENT_RESOLVER_PRIVATE_KEY }}

Override the default bot list with the bots input:

    with:
      bots: "copilot-pull-request-reviewer cursor claude"

Test plan

• Merge this, then open PostHog/posthog PR to switch the caller
• Push to a PR with outdated bot comments and verify threads get resolved