feat(P04): backup/restore — signed tarball (HMAC-SHA256)

internal/backup/backup.go: Backup (tar.gz + HMAC-SHA256 signature,
excludes /run/orca + sockets + WAL/SHM), VerifySignature, Restore
(signature verify + extract + Force flag). internal/cli/backup.go:
orca backup --out + orca restore --in --force. Tests: round-trip,
signature mismatch, exclusion, force-refuse, force-overwrite.

---ci---
project: orca
phase: 04
milestone: v0.11
status: execute
---/ci---

internal/cli/namespace_test.go

@@ -34,6 +34,8 @@ func resetCommandFlags() {
 	stopID, runTarget, runIDKey, jobWatch = "", "", "", false
 	capSetCPU, capSetMem, capSetDisk, capNodeID = 0, 0, 0, ""
 	auditLimit = 50
+	backupOutPath, restoreInPath, restoreTargetDir = "", "", ""
+	restoreForce = false
 	resetNSFlags()
 }