Abort the run when packing fails, even under --skip-errors (closes #161)

A chunk is registered as pending (known, scanner-pending, packer pending-row) before it is packed. Under --skip-errors the scanner skipped a file on any processing error, including a failure inside addChunkToPacker (packing, database, encryption, upload). The pending chunk then stayed queued and a later blob finalize inserted it into the chunks table with no blob_chunks row, so a snapshot could complete holding a file whose chunk is in no blob and cannot be restored.

Errors from addChunkToPacker are now marked and abort the run regardless of --skip-errors; only open and read errors are skipped. The bookkeeping order is unchanged. Flag help and comments now say only unreadable files are skipped.

Model: opus-4-8

internal/cli/root.go

@@ -57,8 +57,9 @@ on the source system.`,
 	cmd.PersistentFlags().BoolVarP(&rootFlags.Quiet, "quiet", "q", false,
 		"Suppress non-error output")
 	cmd.PersistentFlags().BoolVar(&rootFlags.SkipErrors, "skip-errors", false,
-		"Continue past per-file errors instead of aborting "+
-			"(applies to snapshot create and restore)")
+		"Skip files that cannot be read when creating a snapshot, or "+
+			"that cannot be restored when restoring, instead of aborting "+
+			"(packing and storage errors still abort)")
 
 	// Add subcommands
 	cmd.AddCommand(