Refactor: extract duplicated validation and flag logic (bd-g5p7)

- Created internal/validation package for centralized validation logic
- Created cmd/bd/flags.go for shared flag registration
- Updated create and update commands to use shared logic
- Added support for 'P1' style priority to update command
- Added tests for validation logic

Amp-Thread-ID: https://ampcode.com/threads/T-c8d369a3-32f0-42a0-96d1-fd589e89bd6b
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/flags.go

@@ -0,0 +1,17 @@
+package main
+
+import "github.com/spf13/cobra"
+
+// registerCommonIssueFlags registers flags common to create and update commands.
+func registerCommonIssueFlags(cmd *cobra.Command) {
+	cmd.Flags().StringP("assignee", "a", "", "Assignee")
+	cmd.Flags().StringP("description", "d", "", "Issue description")
+	cmd.Flags().String("design", "", "Design notes")
+	cmd.Flags().String("acceptance", "", "Acceptance criteria")
+	cmd.Flags().String("external-ref", "", "External reference (e.g., 'gh-9', 'jira-ABC')")
+}
+
+// registerPriorityFlag registers the priority flag with a specific default value.
+func registerPriorityFlag(cmd *cobra.Command, defaultVal string) {
+	cmd.Flags().StringP("priority", "p", defaultVal, "Priority (0-4 or P0-P4, 0=highest)")
+}