Add Ardent branch system skill

[never2average]

Summary

• add a bundled ardent-branch system skill as an explicit entry point for disposable Ardent branch workflows
• keep the safety contract aligned with db-sandbox: agents receive only branch DATABASE_URL, never source DB credentials
• cover the new bundled skill in the system-skill fingerprint test

Validation

• git diff --check
• PATH=/root/.cargo/bin:$PATH CARGO_TARGET_DIR=/tmp/fpna-thinwedge-sanitized-verify/thinwedge-rs/target cargo test -p thinwedge-skills fingerprint_traverses_nested_entries

Note

This is a skill metadata/workflow addition only. It does not perform live AWS, RDS, or Ardent mutations.

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