Fix config-not-found errors, dev-build hint, unify output writer

ResolveConfigPath now stats explicit paths from --config and
$VAULTIK_CONFIG and produces an actionable error naming the bad
path and suggesting 'vaultik config init' (with the right path
in the --config case). The default-search failure message lists
the paths it tried.

The scanner no longer hard-codes os.Stdout vs io.Discard based on
EnableProgress. ScannerConfig and ScannerParams take an explicit
Output io.Writer, and the Vaultik caller passes v.Stdout — which
itself is set to io.Discard in --cron mode. One knob controls
both scanner-level and Vaultik-level user-facing output.

The version command prints a hint when Version == "dev" telling
the user this is a development build without embedded version
metadata.

internal/vaultik/snapshot.go

@@ -156,6 +156,7 @@ func (v *Vaultik) createNamedSnapshot(opts *SnapshotCreateOptions, hostname, sna
 
 	scanner := v.ScannerFactory(snapshot.ScannerParams{
 		EnableProgress: !opts.Cron,
+		Output:         v.Stdout,
 		Fs:             v.Fs,
 		Exclude:        v.Config.GetExcludes(snapName),
 		SkipErrors:     opts.SkipErrors,