Regenerate all docs from specs

For: Spec Author

A spec author edits .usm files and regenerates every derived artifact, confirming the browser preview stays current.

Part of: Generate Command

Steps

• Run (cli) — usm generate from the repo root
• Review (browser) — regenerated docs in the running usm docs serve preview — hot reload picks up changes
• Commit (cli) — regenerated artifacts only after lint and typecheck pass

How this is verified

• markdown files written for each .usm
• aggregator docs written if system.usm present