Adopt sneak.berlin/go/vaultik vanity import path, README overhaul

Module path changed from git.eeqj.de/sneak/vaultik to
sneak.berlin/go/vaultik (vanity redirect). All imports, ldflags,
Dockerfile, goreleaser config, and docs updated. App data/config
directories now use plain "vaultik" instead of the reverse-DNS name.

README:
- New copy-pasteable quickstart at top: go install, config init,
  age keypair, config set for key + file:// destination, home backup
- All command names in command details are code-quoted
- config set/get gained sequence index support (age_recipients.0)
  so lists are settable from the CLI
- Dockerfile build is CGO_ENABLED=0 to match the pure-Go build

internal/cli/config_test.go

@@ -4,8 +4,8 @@ import (
 	"strings"
 	"testing"
 
-	"git.eeqj.de/sneak/vaultik/internal/config"
 	"gopkg.in/yaml.v3"
+	"sneak.berlin/go/vaultik/internal/config"
 )
 
 // TestDefaultConfigTemplateParses ensures the init template is valid YAML
@@ -45,6 +45,8 @@ func TestDefaultConfigTemplateParses(t *testing.T) {
 
 const testYAML = `# top comment
 compression_level: 3
+age_recipients:
+  - age1aaa
 s3:
   bucket: oldbucket # inline comment
   region: us-east-1
@@ -74,6 +76,9 @@ func TestYAMLPathGet(t *testing.T) {
 		{"compression_level", "3", false},
 		{"s3.bucket", "oldbucket", false},
 		{"s3.region", "us-east-1", false},
+		{"age_recipients.0", "age1aaa", false},
+		{"age_recipients.5", "", true},
+		{"age_recipients.notanumber", "", true},
 		{"s3.nonexistent", "", true},
 		{"nonexistent", "", true},
 		{"compression_level.sub", "", true},
@@ -114,6 +119,17 @@ func TestYAMLPathSet(t *testing.T) {
 		t.Fatalf("set newmap.newkey: %v", err)
 	}
 
+	// Overwrite a sequence element and append a new one
+	if err := yamlPathSet(root, splitPath("age_recipients.0"), "age1bbb"); err != nil {
+		t.Fatalf("set age_recipients.0: %v", err)
+	}
+	if err := yamlPathSet(root, splitPath("age_recipients.1"), "age1ccc"); err != nil {
+		t.Fatalf("append age_recipients.1: %v", err)
+	}
+	if err := yamlPathSet(root, splitPath("age_recipients.5"), "age1ddd"); err == nil {
+		t.Error("expected out-of-range append to fail")
+	}
+
 	// Round-trip and verify values + comment preservation
 	out, err := yaml.Marshal(root)
 	if err != nil {
@@ -121,7 +137,7 @@ func TestYAMLPathSet(t *testing.T) {
 	}
 	text := string(out)
 
-	for _, want := range []string{"newbucket", "s3.example.com", "newkey: val", "# top comment", "# inline comment"} {
+	for _, want := range []string{"newbucket", "s3.example.com", "newkey: val", "# top comment", "# inline comment", "age1bbb", "age1ccc"} {
 		if !contains(text, want) {
 			t.Errorf("round-tripped YAML missing %q:\n%s", want, text)
 		}