Move --type into the search Qualifiers

The --type filter was concatenated into ImmutableKeywords in
SearchQueryBuild, which bypassed search.Query's quoting and let values
that contained quotes (or extra qualifiers) corrupt the final query.
Per babakks's suggestion in the review thread, model it as a regular
qualifier instead.

Add an IssueType field to Qualifiers tagged `qualifier:"type"` so it
shares a key with the existing Type field, and rework Qualifiers.Map
to honour the tag and concatenate values when multiple fields share
the same key. SearchQueryBuild now drops its bespoke type:X
formatting and just sets Qualifiers.IssueType, leaving the
keyword/qualifier escaping to pkg/search.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

pkg/cmd/pr/shared/params_test.go

@@ -173,6 +173,32 @@ func Test_listURLWithQuery(t *testing.T) {
 			want:    "https://example.com/path?q=label%3A%22help+wanted%22+label%3Adocs+milestone%3A%22Codename+%5C%22What+Was+Missing%5C%22%22+state%3Aopen+type%3Apr",
 			wantErr: false,
 		},
+		{
+			name: "issue type",
+			args: args{
+				listURL: "https://example.com/path",
+				options: FilterOptions{
+					Entity:    "issue",
+					State:     "open",
+					IssueType: "Bug",
+				},
+			},
+			want:    "https://example.com/path?q=state%3Aopen+type%3ABug+type%3Aissue",
+			wantErr: false,
+		},
+		{
+			name: "issue type with spaces is quoted",
+			args: args{
+				listURL: "https://example.com/path",
+				options: FilterOptions{
+					Entity:    "issue",
+					State:     "open",
+					IssueType: `Hot "Spicy" Bug`,
+				},
+			},
+			want:    "https://example.com/path?q=state%3Aopen+type%3A%22Hot+%5C%22Spicy%5C%22+Bug%22+type%3Aissue",
+			wantErr: false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {