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/creds.py

@@ -0,0 +1,40 @@
+"""Token file reader.
+
+Format: ``discord_id:password`` on a single line. Whitespace tolerated.
+The Discord ID is the URL path segment under cloud.tm.center/<id>/ that
+restic-rest-server's --private-repos enforces against the basic-auth
+username. The password is the bcrypt'd entry's plaintext AND the restic
+repo encryption password (cloud-svc provisions one password covering both).
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+
+class CredentialsError(Exception):
+    """Raised when the token file is missing or malformed."""
+
+
+def read_credentials(token_file: Path) -> tuple[str, str]:
+    if not token_file.exists():
+        raise CredentialsError(
+            f"cloud-sync token not found at {token_file}. "
+            f"After /register in Discord you should have received credentials; "
+            f"paste them into this file as 'discord_id:password' on one line."
+        )
+    raw = token_file.read_text(encoding="utf-8").strip()
+    if ":" not in raw:
+        raise CredentialsError(
+            f"cloud-sync token at {token_file} malformed "
+            f"(expected 'discord_id:password' on one line)"
+        )
+    discord_id, password = raw.split(":", 1)
+    discord_id = discord_id.strip()
+    password = password.strip()
+    if not discord_id or not password:
+        raise CredentialsError(
+            f"cloud-sync token at {token_file} malformed "
+            f"(empty discord_id or password)"
+        )
+    return discord_id, password