fix: improve priority format documentation and error message (GH#517)

Claude was using word-based priorities like "medium" instead of numeric
values (0-4 or P0-P4), causing bd create commands to fail in a loop.

Changes:
- Update bd prime output to clearly document priority format with example
- Add explicit note: NOT "high"/"medium"/"low"
- Improve error message to mention that words are not valid

Closes #517

🤖 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