Feature: Detect PRD md/json out of sync and offer regeneration

[MiniCodeMonkey]

Context

Users sometimes edit prd.md directly (via vim, Claude, etc.) and want the JSON to reflect those changes. The current workflow for regenerating JSON from an edited markdown file is unclear and error-prone.

Problem

• No clear way to regenerate prd.json from an edited prd.md
• chief edit <name> followed by immediately exiting Claude can produce incomplete JSON (see related merge/truncation bugs)
• Users have to guess the workflow: edit md → chief edit → exit → hope conversion works
• The docs at chiefloop.com/concepts/prd-format.html don't cover this workflow

Suggested solution

• Detect drift: Chief could notice when prd.md has been modified more recently than prd.json and prompt to rebuild
• CLI command: chief rebuild <name> or chief sync <name> to programmatically regenerate JSON from markdown without going through Claude
• Document the workflow on the PRD format docs page