Add automated weekly rebase workflow for opensoft-prod branch

[Copilot]

Automates the weekly maintenance of rebasing opensoft-prod onto production, with automatic conflict notifications to maintainers.

Changes

• .github/workflows/rebase-opensoft-prod.yml: Weekly workflow (Sundays 2:00 AM UTC) that attempts rebase, force-pushes on success, or creates labeled issue on conflict
• .github/rebase-conflict-message.md: Conflict resolution guide with step-by-step git commands for manual intervention

Workflow behavior

# Runs weekly or on-demand
on:
  schedule:
    - cron: "0 2 * * 0"
  workflow_dispatch:

# Success path: clean rebase → force-push with --force-with-lease
# Conflict path: abort rebase → create issue with resolution instructions

The workflow never auto-merges conflicts—it aborts and delegates to maintainers via GitHub issue.

Original prompt

Create a GitHub Actions workflow in .github/workflows/rebase-opensoft-prod.yml that will:

• Run weekly (scheduled, e.g. Sunday night) or via 'workflow_dispatch' manual trigger.
• Attempt to rebase the opensoft-prod branch onto the production branch.
• If the rebase succeeds (no conflicts), force-push opensoft-prod to origin.
• If there are conflicts, abort the rebase and open a GitHub issue alerting maintainers, using .github/rebase-conflict-message.md for the issue body.
• Do not attempt auto-merging of conflicts, just notify maintainers to resolve them.

Also, add .github/rebase-conflict-message.md containing instructions for manual conflict resolution.

Do not modify any other files or workflows.

This pull request was created from Copilot chat.

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