fix(prime): document priority format to prevent Claude using 'medium'

GH#517: Claude uses 'medium' instead of P2/2 for priority, causing
infinite error loops. The bd prime output didn't show the --priority
flag or explain the valid format.

Fix: Add --priority=2 to the create example and add a clear note
explaining that priority must be 0-4 or P0-P4, NOT 'high/medium/low'.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/prime.go

@@ -236,7 +236,8 @@ bd sync                     # Push to remote
 - ` + "`bd show <id>`" + ` - Detailed issue view with dependencies
 
 ### Creating & Updating
-- ` + "`bd create --title=\"...\" --type=task|bug|feature`" + ` - New issue
+- ` + "`bd create --title=\"...\" --type=task|bug|feature --priority=2`" + ` - New issue
+  - Priority: 0-4 or P0-P4 (0=critical, 2=medium, 4=backlog). NOT "high"/"medium"/"low"
 - ` + "`bd update <id> --status=in_progress`" + ` - Claim work
 - ` + "`bd update <id> --assignee=username`" + ` - Assign to someone
 - ` + "`bd close <id>`" + ` - Mark complete