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>

.beads/.gitignore

@@ -30,3 +30,5 @@ beads.right.meta.json
 !issues.jsonl
 !metadata.json
 !config.json
+deletions.jsonl
+deletions.jsonl.migrated