Collapse snapshot prune into vaultik prune; auto-clean on removal

The CLI had two commands named "prune" doing different jobs (local
DB orphan cleanup vs. remote blob garbage collection), which was
confusing and forced a manual two-step workflow after deleting any
snapshot.

Single user-facing prune surface is now `vaultik prune`, which calls
PruneDatabase (local orphan cleanup) then PruneBlobs (remote unref
blob GC). Snapshot deletion paths (snapshot remove, snapshot remove
--all, snapshot purge) auto-run CleanupOrphanedData inline so the
local index database doesn't accumulate ghost rows after every
removal — the user observed ~39k orphaned files and 2 orphaned blobs
after a remove --all because that cleanup was previously a separate
opt-in command. `snapshot prune` is removed.

Also addresses the doc/help-string drift the user audit caught:

  * cli/prune.go help text used to reference a non-existent
    `vaultik purge` command.
  * cli/config.go get/set short/long examples were S3-specific
    (s3.bucket) when the primary storage configuration is
    storage_url.
  * vaultik/info.go printed S3 Bucket/Endpoint/Region labels
    unconditionally; for file:// or rclone:// users those rows
    were empty. The Storage Configuration block now prints the
    storer's Type+Location first, the storage_url string when set,
    and only emits S3 rows that are actually populated.
  * vaultik/info.go's "Run 'vaultik prune --remote'" hint
    referenced a flag that doesn't exist.
  * vaultik/blobcache.go's doc comment claimed LRU eviction, which
    is no longer the restore-time policy (the sweeper drives
    eviction; LRU is the safety-net fallback when maxBytes is
    finite).
  * README.md listed `vaultik restore`, `vaultik snapshot prune`,
    and `s3.bucket` example, all out of date.

README's roadmap section is rewritten with concrete pre-1.0 items
(security audit, error-condition tests, parallel blob downloads,
restart of interrupted restore, …) so the next-steps surface
matches what the project actually still needs.

The cleanup calls are guarded against a nil SnapshotManager so
tests that construct a bare Vaultik struct continue to work.

internal/vaultik/info.go

@@ -22,14 +22,29 @@ func (v *Vaultik) ShowInfo() error {
 	v.printfStdout("Go Version:      %s\n", runtime.Version())
 	v.printlnStdout()
 
-	// Storage Configuration
+	// Storage Configuration. The backend is selected by storage_url
+	// (s3://, file://, rclone://); the legacy s3.* fields are only
+	// printed when they're actually populated, since the URL scheme
+	// is the primary configuration.
 	v.printfStdout("=== Storage Configuration ===\n")
-	v.printfStdout("S3 Bucket:       %s\n", v.Config.S3.Bucket)
+	storageInfo := v.Storage.Info()
+	v.printfStdout("Type:            %s\n", storageInfo.Type)
+	v.printfStdout("Location:        %s\n", storageInfo.Location)
+	if v.Config.StorageURL != "" {
+		v.printfStdout("Storage URL:     %s\n", v.Config.StorageURL)
+	}
+	if v.Config.S3.Bucket != "" {
+		v.printfStdout("S3 Bucket:       %s\n", v.Config.S3.Bucket)
+	}
 	if v.Config.S3.Prefix != "" {
 		v.printfStdout("S3 Prefix:       %s\n", v.Config.S3.Prefix)
 	}
-	v.printfStdout("S3 Endpoint:     %s\n", v.Config.S3.Endpoint)
-	v.printfStdout("S3 Region:       %s\n", v.Config.S3.Region)
+	if v.Config.S3.Endpoint != "" {
+		v.printfStdout("S3 Endpoint:     %s\n", v.Config.S3.Endpoint)
+	}
+	if v.Config.S3.Region != "" {
+		v.printfStdout("S3 Region:       %s\n", v.Config.S3.Region)
+	}
 	v.printlnStdout()
 
 	// Backup Settings
@@ -337,7 +352,7 @@ func (v *Vaultik) printRemoteInfoTable(result *RemoteInfoResult) {
 		humanize.Comma(int64(result.OrphanedBlobCount)), humanize.Bytes(uint64(result.OrphanedBlobSize)))
 
 	if result.OrphanedBlobCount > 0 {
-		v.printfStdout("\nRun 'vaultik prune --remote' to remove orphaned blobs.\n")
+		v.printfStdout("\nRun 'vaultik prune' to remove orphaned blobs.\n")
 	}
 }