Automate synchronization from the Accelerate upstream

[danielsaeuberli]

Adds a GitHub-native pull-based synchronization workflow.

Behaviour:

• checks Accelerate-GmbH/industry-function-graph every 5 minutes and on manual dispatch;
• mirrors the upstream tree into a dedicated sync/accelerate-main branch only when drift exists;
• preserves the DIDAS Pages workflow and the synchronization workflow;
• runs the graph validation, acceptance queries and generated-output drift check before opening/updating a PR;
• records a successful commit status on the synchronized commit;
• never writes directly to main.

Uses GitHub Actions, git and the GitHub CLI only; no external service or cross-organisation secret is required.