Test the remaining CLI cases (closes #16)

Most of the command-line contract was already tested through run. This
adds what was missing: report and trees with no database exit 1 with
the message telling the user to run scan; a scan that skips an
unreadable file prints its warning and counts the skip in its summary;
the report and trees summary lines are checked exactly. The scan tests
now capture the process's own stdout, which scan would write to
directly, so a stray stdout write in scan fails them. The fatal-path
test takes its subcommands from the command tree, so a new subcommand
wired without runE fails it. The nonexistent-operand test gets an
accurate name.

Model: opus-5-5

cancel_test.go

@@ -344,13 +344,14 @@ func storedPaths(t *testing.T, path string) []string {
 
 // TestRunScanInterrupted calls the scan entrypoint with a context that
 // is already cancelled, as when a signal arrives at once. It must return
-// errInterrupted promptly with its one line on stderr, leave the
-// database valid and as it was, and leave nothing in the way of the
-// next scan, which must bring the database up to date.
+// errInterrupted promptly with its one line on stderr and nothing on
+// stdout, leave the database valid and as it was, and leave nothing in
+// the way of the next scan, which must bring the database up to date.
 func TestRunScanInterrupted(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
 
+	stdout := captureStdout(t)
 	stderr := captureStderr(t)
 	dir := buildSmokeTree(t)
 
@@ -393,6 +394,10 @@ func TestRunScanInterrupted(t *testing.T) {
 		t.Errorf("stderr = %q, want %q", got, want)
 	}
 
+	if got := stdout(); got != "" {
+		t.Errorf("stdout = %q, want nothing (data only)", got)
+	}
+
 	assertNoSidecars(t, path)
 
 	if got := storedPaths(t, path); !slices.Equal(got, before) {
@@ -415,12 +420,13 @@ func TestRunScanInterrupted(t *testing.T) {
 // TestRunScanInterruptedMidHash interrupts the scan entrypoint part-way
 // through its hash phase, after the database is open. It must return
 // errInterrupted, release the lock, end stderr with its line counting
-// every file the walk reached, close the database out of WAL mode, and
-// keep the records it hashed.
+// every file the walk reached, write nothing to stdout, close the
+// database out of WAL mode, and keep the records it hashed.
 func TestRunScanInterruptedMidHash(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
 
+	stdout := captureStdout(t)
 	stderr := captureStderr(t)
 	dir := buildWalkCancelTree(t)
 
@@ -438,6 +444,10 @@ func TestRunScanInterruptedMidHash(t *testing.T) {
 		t.Errorf("stderr = %q, want it to end with %q", got, want)
 	}
 
+	if got := stdout(); got != "" {
+		t.Errorf("stdout = %q, want nothing (data only)", got)
+	}
+
 	assertNoSidecars(t, path)
 
 	db, err := openReportDatabase(t.Context(), path)