docs: add Lovable sync setup + drift recovery runbook

[adm01-debug]

Summary

Adds a runbook documenting the Lovable ↔ GitHub sync configuration and how to recover from the kind of drift that caused the 2026-04 incident.

Why

On 2026-04-23 the lovable-dev[bot] started pushing edits to lovable-sync-* side branches instead of main. Over 4 days this produced:

• 9 stale lovable-sync-* PRs
• 2540 commits accumulated off-main
• Every open dependabot PR (build(deps): bump react-helmet-async from 2.0.5 to 3.0.0 #2, build(deps): bump date-fns from 2.30.0 to 4.1.0 #4, build(deps): bump framer-motion from 11.18.2 to 12.34.5 #5, build(deps): bump vitest from 3.2.4 to 4.1.0 #7, chore(deps)(deps-dev): bump postcss from 8.5.10 to 8.5.12 in the development-minor group across 1 directory #15) became unmergeable because their base diverged
• One orphaned UX PR (Improve UX navigation and accessibility #11) with no common ancestor with either line

Recovery required force-pushing main to the latest lovable-sync HEAD and closing 11 stale PRs. There was no documented config or runbook, so anyone hit by the same misconfig would have to reverse-engineer the investigation.

What's in the doc

docs/LOVABLE_SYNC_SETUP.md (222 lines):

1. Postmortem timeline — UTC-precise reconstruction of the 2026-04 incident
2. Correct Lovable configuration — target branch must be main, mode push, what to avoid
3. Detection — stub .github/workflows/check-lovable-drift.yml that fails CI if any lovable-sync-* branch is older than 24 h with unique commits (catches recurrence in 1 day instead of 4)
4. Recovery runbook — 6 steps to follow if drift recurs, with explicit commands and warnings about the destructive force-push step

Test plan

• Read through the doc
• (Optional) Adopt the workflow stub from §3 to enforce drift detection in CI

https://claude.ai/code/session_01KWeDG

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✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch claude/lovable-sync-config-docs

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