pivot to Python: replace Kotlin/JVM with stdlib zipapp

Reasons stacked up:
  - AV: unsigned JARs that auto-download binaries + upload files trigger
    Windows Defender false-positives more often than Python scripts
    invoked by code-signed python.exe.
  - Qt UI option: PySide6 opens a path to a real Qt UI (matching Prism's
    look) if needed later. JVM Qt bindings are abandoned.
  - frazclient already needs Python; inlining as 'import cloud_sync' is
    zero overhead vs the launcher always shelling out to java.

Implementation:
  - cloud_sync package: cli.py (argparse), creds.py, scope.py,
    restic.py (binary discovery + auto-download + sha256 verify),
    sync.py (pull/push subprocess restic).
  - pyproject.toml with hatchling backend; pip-installable.
  - Makefile builds cloud-sync.pyz via python -m zipapp (~53 KB).
  - 33 pytest tests, stdlib only on runtime.
  - CI workflow runs pytest matrix (3.10/3.11/3.12) + builds pyz.
  - DESIGN.md + README.md updated to reflect Python.

E2E verified against local restic-rest-server:
  pull empty → push initial → rm -rf local → pull restores → modify+push
  creates second snapshot → client forget --prune blocked by --append-only.

Throws away ~565 LOC of Kotlin (and 18 jar tests) committed earlier in
this same session. Net result is ~250 LOC Python + 33 tests = smaller
and more aligned with the rest of the stack.

cloud_sync/__main__.py

@@ -0,0 +1,11 @@
+"""Entry for ``python -m cloud_sync`` and the zipapp build."""
+
+from __future__ import annotations
+
+import sys
+
+from .cli import main
+
+
+if __name__ == "__main__":
+    sys.exit(main())