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/snapshot/backup_test.go

@@ -13,8 +13,8 @@ import (
 	"testing/fstest"
 	"time"
 
-	"git.eeqj.de/sneak/vaultik/internal/database"
-	"git.eeqj.de/sneak/vaultik/internal/types"
+	"sneak.berlin/go/vaultik/internal/database"
+	"sneak.berlin/go/vaultik/internal/types"
 )
 
 // MockS3Client is a mock implementation of S3 operations for testing