Add weekly upstream dev-sync workflow (autonomous)

[Sjoerd-Bo3]

Summary

A durable, autonomous weekly GitHub Actions workflow that syncs upstream dev into Build-Experiment on its own — no Claude session or local machine needed.

It targets the dev branch because GitHub only runs schedule: workflows from the default branch, even though the job operates on Build-Experiment.

What it does each Monday

• Nothing new upstream → no-op.
• New commits, clean merge → pushes a chore/auto-dev-sync branch, opens a PR to Build-Experiment, and enables auto-merge so it merges once the compile check passes.
• New commits, merge conflict → opens an issue asking for a manual merge. It never leaves a half-merged state.

This matches the manual dev-sync we've been doing, and preserves fork customizations (the merge keeps our code; only genuinely-conflicting upstream changes stop it for review).

⚠️ One-time setup (only you can do these)

Without these it still opens the weekly PR — you'd just merge by hand:

1. SYNC_PAT secret — a fine-grained PAT with contents:write + pull-requests:write. Required because pushes made with the default GITHUB_TOKEN do not trigger other workflows, so the compile check would never run on the sync branch and auto-merge could never see green.
2. Settings → Pull Requests → enable "Allow auto-merge".
3. Branch protection on Build-Experiment requiring the "Build for Simulator" check — this is what makes auto-merge wait for the compile check instead of merging immediately.

Notes

• Recurring cron picked at 23 6 * * 1 (off the :00 mark).
• Still manually runnable via workflow_dispatch.
• No upstream PR numbers/authors are referenced in the branch, commits, or PR body.

🤖 Generated with Claude Code

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