cli: scaffold phase 1 asset release commands

Add metin-release CLI with argparse dispatcher, result envelope and
error hierarchy, and the full Phase 1 release subcommand set:

  release inspect         - scan source root
  release build-manifest  - wraps make-manifest.py
  release sign            - wraps sign-manifest.py, enforces mode 600
  release diff-remote     - HEAD each blob hash against a base URL
  release upload-blobs    - rsync release dir minus manifest
  release promote         - rsync manifest.json + signature
  release verify-public   - GET + Ed25519 verify, optional blob sampling
  release publish         - composite of the above with per-stage timings

Respects --json / --verbose / --quiet. Exit codes follow the plan
(1 validation, 2 remote, 3 integrity, 4 reserved for ERP).

src/metin_release/commands/__init__.py

@@ -0,0 +1,23 @@
+from __future__ import annotations
+
+from . import (
+    build_manifest,
+    diff_remote,
+    inspect,
+    promote,
+    publish,
+    sign,
+    upload_blobs,
+    verify_public,
+)
+
+__all__ = [
+    "build_manifest",
+    "diff_remote",
+    "inspect",
+    "promote",
+    "publish",
+    "sign",
+    "upload_blobs",
+    "verify_public",
+]