refactor: consolidate GitHub integration into dojo/github/ package

[Maffooch]

Summary

• Consolidate scattered GitHub integration code (dojo/github.py, dojo/github_issue_link/, GitHub forms in dojo/forms.py, GitHub models in dojo/models.py, GitHub templates) into a single self-contained dojo/github/ package.
• Match the canonical dojo/url/ reference layout from CLAUDE.md: __init__.py + admin.py + models.py + services.py + ui/{__init__,forms,urls,views}.py + templates/dojo/.
• Extract the inline Github(api_key).get_user() credential check from new_github view into services.validate_github_credentials() per CLAUDE.md Phase 2 ("external integration calls belong in services.py").
• Backward-compat re-exports left in dojo/models.py (5 GITHUB models) and dojo/forms.py (6 GITHUB forms) per the playbook. dojo/utils.py and dojo/urls.py updated to use the new canonical paths.
• GitHub Enterprise OAuth env vars in settings.dist.py left in place (out of scope per task; conceptually belong with SSO).
• No behavior change. No DB migrations (single dojo app preserves app_label).

Test plan

• python manage.py check — System check identified no issues
• python manage.py makemigrations --check --dry-run — No changes detected
• python manage.py test unittests -k github — 29/29 tests pass
• Backward-compat re-exports resolve from dojo.models and dojo.forms (model/form identity matches new canonical paths)
• New canonical paths resolve: dojo.github.{models,services}, dojo.github.ui.{forms,urls}
• Templates resolve from dojo/github/templates/dojo/{github,new_github,delete_github}.html
• Admin registrations active for GITHUB_Conf, GITHUB_Issue, GITHUB_PKey, GITHUB_Clone, GITHUB_Details_Cache
• ruff check clean on all touched files
• Smoke-test in dev container with enable_github=True: /github, /github/add, /github/<id>/delete; create finding linked to GITHUB_PKey-configured product, push to GitHub, close, reopen

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