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

.goreleaser.yaml

@@ -20,8 +20,8 @@ builds:
       - arm64
     ldflags:
       - -s -w
-      - -X 'git.eeqj.de/sneak/vaultik/internal/globals.Version={{ .Version }}'
-      - -X 'git.eeqj.de/sneak/vaultik/internal/globals.Commit={{ .Commit }}'
+      - -X 'sneak.berlin/go/vaultik/internal/globals.Version={{ .Version }}'
+      - -X 'sneak.berlin/go/vaultik/internal/globals.Commit={{ .Commit }}'
 
 archives:
   - id: default