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>

internal/validation/bead.go

@@ -52,7 +52,7 @@ func ParseIssueType(content string) (types.IssueType, error) {
 func ValidatePriority(priorityStr string) (int, error) {
 	priority := ParsePriority(priorityStr)
 	if priority == -1 {
-		return -1, fmt.Errorf("invalid priority %q (expected 0-4 or P0-P4)", priorityStr)
+		return -1, fmt.Errorf("invalid priority %q (expected 0-4 or P0-P4, not words like high/medium/low)", priorityStr)
 	}
 	return priority, nil
 }