fix(ci): resolve lint and test failures

Fix two CI failures that were blocking main:

1. Lint error in cmd/bd/onboard.go:126
   - Unchecked fmt.Fprintf return value
   - Fixed by explicitly ignoring with _, _

2. Test failures in internal/storage/sqlite
   - TestCreateIssues/duplicate_ID_error was passing but
     TestCreateIssuesRollback/rollback_on_conflict_with_existing_ID failed
   - Root cause: CreateIssues used INSERT OR IGNORE which silently
     ignored duplicate IDs instead of returning an error
   - Fixed by adding duplicate ID detection in EnsureIDs():
     a) Check for duplicates within the batch
     b) Check for conflicts with existing database IDs

Both fixes are minimal and targeted to unblock CI.

cmd/bd/onboard.go

@@ -123,7 +123,7 @@ The old approach of embedding full instructions in AGENTS.md is deprecated
 because it wasted tokens and got stale when bd upgraded.`,
 	Run: func(cmd *cobra.Command, args []string) {
 		if err := renderOnboardInstructions(cmd.OutOrStdout()); err != nil {
-			fmt.Fprintf(cmd.ErrOrStderr(), "Error: %v\n", err)
+			_, _ = fmt.Fprintf(cmd.ErrOrStderr(), "Error: %v\n", err)
 		}
 	},
 }