fix: tighten error classification and pull request selection

Only Gitea's archived-repository messages mean the repository is archived,
so an archived label is no longer reported as one. Refused POSTs to the
organization, team, user and SSH key collections name the resource they would
have created, and the existing pull request is looked up with escaped branch
names.

pullSelector without an argument refuses an explicit --repo, like gh: the
checked-out branch says nothing about another repository.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/ref_test.go

@@ -135,6 +135,11 @@ func TestPullSelectorDefaultsToTheCheckedOutBranch(t *testing.T) {
 			require.Equal(t, tc.want, n)
 		})
 	}
+	a, _ := refApplication(t, branch("feat/two", nil))
+	a.options.repo = "alice/project"
+	_, _, err := a.pullSelector(commandContext(), nil)
+	require.Equal(t, clierr.Usage, clierr.ExitCode(err))
+	require.EqualError(t, err, "pass a pull request number, URL or branch with --repo")
 }
 
 func TestPullSelectorAcceptsNumbersURLsAndBranches(t *testing.T) {