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/config/config.go

@@ -9,13 +9,13 @@ import (
 
 	"filippo.io/age"
 	"git.eeqj.de/sneak/smartconfig"
-	"git.eeqj.de/sneak/vaultik/internal/log"
 	"github.com/adrg/xdg"
 	"go.uber.org/fx"
 	"gopkg.in/yaml.v3"
+	"sneak.berlin/go/vaultik/internal/log"
 )
 
-const appName = "berlin.sneak.app.vaultik"
+const appName = "vaultik"
 
 // expandTilde expands ~ at the start of a path to the user's home directory.
 func expandTilde(path string) string {