Refactor: Extract duplicated validation logic to internal/validation

Extracted repeated priority and ID validation patterns from CLI commands
into reusable functions in internal/validation/bead.go.

Changes:
- Added ValidatePriority(): Combines parsing and error handling
- Added ValidateIDFormat(): Validates ID format and extracts prefix
- Added ValidatePrefix(): Validates prefix matching with database config
- Updated create.go and show.go to use new validation functions
- Simplified force flag logic to always call ValidatePrefix()
- Added comprehensive tests for all validation functions
- Added TODO comment for daemon mode validation enhancement

Results:
- Reduced code duplication by ~20 lines
- Centralized validation logic for easier maintenance
- Consistent error messages across all commands
- All tests passing

Fixes bd-g5p7

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

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

cmd/bd/show.go

@@ -345,9 +345,9 @@ var updateCmd = &cobra.Command{
 		}
 		if cmd.Flags().Changed("priority") {
 			priorityStr, _ := cmd.Flags().GetString("priority")
-			priority := validation.ParsePriority(priorityStr)
-			if priority == -1 {
-				fmt.Fprintf(os.Stderr, "Error: invalid priority %q (expected 0-4 or P0-P4)\n", priorityStr)
+			priority, err := validation.ValidatePriority(priorityStr)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 				os.Exit(1)
 			}
 			updates["priority"] = priority