test: audit and clean up test suite

Delete tautological, no-assertion, and permanently-skipped tests.
Rewrite false-confidence tests that couldn't detect the bugs they
claimed to test. Decouple brittle assertions from error message
strings and third-party library output. Fix misleading test names
and error messages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/init_test.go

@@ -4,7 +4,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"strings"
 	"testing"
 )
 
@@ -90,9 +89,6 @@ func TestInitWriteError(t *testing.T) {
 	if err == nil {
 		t.Error("expected error when writing to invalid path, got nil")
 	}
-	if err != nil && !strings.Contains(err.Error(), "failed to create") {
-		t.Errorf("expected 'failed to create' error, got: %v", err)
-	}
 }
 
 // TestInitExistingFile tests that Init overwrites existing files