Detect migration conflicts and apply descriptive labels with resolution guidance

[Copilot]

The workflow previously only added a generic "migrations" label. It now detects Django migration number conflicts and applies semantic labels with automated resolution guidance.

Changes

Conflict Detection

• Fetch migration files from PR and base branch via GitHub API
• Extract migration numbers (e.g., 0100 from 0100_add_field.py)
• Compare PR migration numbers against base branch to detect conflicts

Semantic Labeling

• migration: sequence-ok (green) when migrations don't conflict
• migration: clash (red) when migration numbers collide with base branch
• Remove outdated labels including legacy migrations label

Automated Resolution Guidance

• Post comment when conflicts detected listing all conflicting files
• Include makemigrations --merge instructions and alternatives
• Use pagination to prevent duplicate comments

Example

When a PR adds website/migrations/0100_add_field.py but base already has 0100_initial.py:

⚠️ Migration Conflict Detected

- website/migrations/0100_add_field.py conflicts with website/migrations/0100_initial.py

### How to resolve:
1. Delete the conflicting migration file(s) from your branch
2. Pull the latest changes from the base branch
3. Run `python manage.py makemigrations --merge` to create a merge migration
4. Commit and push the merge migration

Files Modified

• .github/workflows/add-migrations-label.yml - Conflict detection and labeling logic
• docs/github_actions.md - Updated workflow documentation

Original prompt

For the github action that adds the migration label instead of onltbadfing the label mske the label say migration: sequence-ok or migration: clash (tyen addca comment to bunp the migration number and dependency.

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