Re-scan after enriching specs

For: Spec Author

A spec author re-runs usm scan after editing .usm files and confirms smart-merge kept their work.

Part of: Scan Command

Steps

• Edit (spec) — summaries, intent, decisions, contracts in .usm files
• Run (cli) — usm scan again
• Verify (spec) — human-edited fields survived the merge; only mechanical fields ($last_updated, paths) changed

How this is verified

• human-edited summary preserved
• mechanical fields like $last_updated updated