Delete dead code and stale fixtures, fix config set reindent (closes #70)

Remove the unused internal/models package (and its coverage-only test),
internal/cli/vaultik_snapshot_types.go (a second, dead SnapshotInfo that
shadowed the live type in internal/vaultik), and two fixtures
(test-config.yml, test/integration-config.yml) that use source_dirs and
other keys config.Config no longer has. All confirmed by grep against
origin/next; none has a production consumer.

config set now writes via yaml.NewEncoder with 2-space indent, matching
defaultConfigTemplate, so the first set no longer reindents the file it
promises to preserve. A test covers the round-trip.

Single commit rather than one commit per deletion, per task instruction.
test/integration-config.yml removed per the manager note on the issue.

Model: opus-4-8

internal/cli/config_test.go

@@ -188,6 +188,47 @@ func TestYAMLPathSet(t *testing.T) {
 	}
 }
 
+// TestConfigSetPreservesFormatting asserts the `config set` write path
+// (marshalConfigYAML) round-trips a 2-space-indented file without reindenting
+// it to yaml.Marshal's 4-space default, and keeps comments.
+func TestConfigSetPreservesFormatting(t *testing.T) {
+	t.Parallel()
+
+	root := parseTestYAML(t)
+
+	err := yamlPathSet(root, splitPath("s3.bucket"), "newbucket")
+	if err != nil {
+		t.Fatalf("set s3.bucket: %v", err)
+	}
+
+	out, err := marshalConfigYAML(root)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	text := string(out)
+
+	for _, want := range []string{"# top comment", "# inline comment"} {
+		if !contains(text, want) {
+			t.Errorf("round-tripped YAML dropped comment %q:\n%s", want, text)
+		}
+	}
+
+	// Nested map keys stay at 2-space indent; the bug reindented them to 4.
+	if !contains(text, "\n  bucket: newbucket") {
+		t.Errorf("expected 2-space indent for s3.bucket, got:\n%s", text)
+	}
+
+	if contains(text, "\n    bucket:") {
+		t.Errorf("s3.bucket reindented to 4 spaces:\n%s", text)
+	}
+
+	// Sequence items under a key also stay at 2 spaces.
+	if !contains(text, "\n  - age1aaa") {
+		t.Errorf("expected 2-space indent for sequence item, got:\n%s", text)
+	}
+}
+
 func splitPath(s string) []string {
 	return strings.Split(s, ".")
 }