ci: add conflict-check reusable workflow

[sydorovdmytro]

Summary

• Extracts the conflict-check workflow as a reusable workflow_call workflow
• Source: identical workflow in loft-enterprise and vcluster (detects unresolved merge conflict markers in PRs targeting release branches)
• SHA-pinned actions/checkout@v6.0.2
• Added persist-credentials: false on checkout
• Parameterized GH_ACCESS_TOKEN as a workflow_call secret (gh-access-token)
• Branch filters stay in callers (enterprise: release-** + v**, vcluster: v** only)
• PR comment condition uses github.event.pull_request.number instead of github.event_name to support both pull_request and pull_request_target callers
• Added concurrency group with cancel-in-progress: true
• Added timeout-minutes: 5 on the job

Caller example

name: Check for unresolved conflicts

on:
  pull_request:
    branches:
      - 'release-**'
      - 'v**'

jobs:
  conflict-check:
    uses: loft-sh/github-actions/.github/workflows/conflict-check.yaml@conflict-check/v1
    secrets:
      gh-access-token: ${{ secrets.GH_ACCESS_TOKEN }}

Test plan

• actionlint passes
• zizmor passes
• CI green
• Tag conflict-check/v1 after merge
• Migration PRs in loft-enterprise and vcluster

Part of the Phase 3 GitHub Actions centralization effort.