[WIP] 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

‼️ unsure: In ee/backend, codex suggested imports of github.com/diggerhq/digger/ee/drift/middleware with github.com/diggerhq/digger/backend/middleware. It indeed looks odd to use drift middleware there; but I thought I'd keep it as is, probably there is a reason for that

🧠 AI Assistance Disclosure Policy

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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• Contributors must understand and take responsibility for any AI-assisted code.
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