Promote Drift to CE

[ZIJ]

This promotes Drift functionality from EE to CE

• what was in /ee/drift now lives in top-level /drift dir
• ee/drift remains but as a thin wrapper importing from CE. main.go is duplicated, I think that's OK
• GitHub Issues notifier from ``ee/cli/pkg/drift/github_issue.go` moved to CE CLI; ee CLI now imports it

Tested, working: https://github.com/diggerhq/test-drift-ce/issues/1

In ee/backend, imports of github.com/diggerhq/digger/ee/drift/middleware replaced with github.com/diggerhq/digger/backend/middleware.

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Important

Inspired by ghostty.
If you used any AI assistance while contributing to Digger, you must disclose it in this PR.

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✅ AI Disclosure Checklist

• I understand that all AI assistance must be disclosed.
• I did not use AI tools in this contribution.
• I used AI tools and have disclosed details below.

Details (if applicable):

Used Codex heavily for this refactor
See agent-tasks/promote-drift-ee-ce.md for plan and implementation notes

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💡 Notes

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• Contributors must understand and take responsibility for any AI-assisted code.
• Failure to disclose is considered disrespectful to maintainers and may delay review.

[motatoes]

we should get rid of this entire package I believe it has all moved to ce

[ZIJ]

i left it as a thin wrapper on purpose (main.go as it was, the rest imported from CE) - so that the "EE build" contains all the same components as CE build (backend svc, cli, drift svc). this way an EE user would have a consistent package, and it leaves room for future EE-only features in the drift svc

[ZIJ]

in the limit of complete EE = CE parity I think we'd still want a separate entry point for each EE component, even if they are functionally identical

[motatoes]

lowercase file name better to be consistent with all others

[ZIJ]

fixed!