feat: 'disable' renames to sync.json.disabled; new 'enable' rename-back

Pause/resume sync without losing the instance_id.

  disable  sync.json -> sync.json.disabled
  enable   sync.json.disabled -> sync.json

Re-enabling preserves the original ULID + url so the same restic
repo continues. No new instance_id minted, no orphaned snapshot
history. Tradeoff vs the previous 'disable = delete' semantics:
the on-disk artifact survives, so a truly fresh start now needs
'disable && rm sync.json.disabled' before 'setup'.

Implementation:

  config.disable(pack)  os.rename(sync.json -> sync.json.disabled).
                        False if no sync.json.
  config.enable(pack)   os.rename(sync.json.disabled -> sync.json).
                        Refuses if sync.json already exists
                        (caller must disable first).
  config.delete(pack)   now sweeps BOTH forms (escape hatch / tests).

setup_flow gains a precheck: if sync.json.disabled is present, point
the user at 'enable' instead of silently minting a fresh ULID over
their existing instance.

Opt-in gate (cfgmod.exists) is unchanged — only literal sync.json
counts. The .disabled sibling is invisible to pull/push, so the
silent-no-op behavior for paused instances Just Works.

cli adds 'enable' subcommand alongside 'disable'. _run_disable prints
'already disabled' when called twice; _run_enable refuses to clobber
an active config (exits 2 with the FileExistsError message).

7 new tests for disable/enable behavior + edge cases (idempotency,
nothing-to-X, refuse-clobber). 80 tests total.

cloud_sync/config.py

@@ -88,16 +88,62 @@ def write(pack_folder: Path, cfg: SyncConfig) -> None:
     p.chmod(0o644)
 
 
-def delete(pack_folder: Path) -> bool:
-    """Remove sync.json. Used by the ``disable`` subcommand. Returns True if a
-    file was removed, False if nothing was there."""
-    p = config_path(pack_folder)
-    if not p.exists():
+def disabled_path(pack_folder: Path) -> Path:
+    return pack_folder / ".cloud-sync" / "sync.json.disabled"
+
+
+def disable(pack_folder: Path) -> bool:
+    """Rename sync.json → sync.json.disabled. Reversible via :func:`enable`.
+
+    Preserves instance_id + url so re-enabling the instance later picks
+    back up the same restic repo (no new ULID minted, no orphaned
+    snapshot history). Returns True if a file was renamed; False if
+    sync.json wasn't there (already off — silent no-op).
+
+    If sync.json.disabled already exists, it's overwritten by the move —
+    that case means the user disabled, then somehow re-ran setup which
+    minted a fresh sync.json; the older disabled file is stale.
+    """
+    src = config_path(pack_folder)
+    if not src.exists():
         return False
-    p.unlink()
+    dst = disabled_path(pack_folder)
+    src.replace(dst)
     return True
 
 
+def enable(pack_folder: Path) -> bool:
+    """Rename sync.json.disabled → sync.json. Inverse of :func:`disable`.
+
+    Returns True if a file was renamed; False if .disabled wasn't there
+    (nothing to re-enable). Refuses if sync.json already exists (would
+    clobber an active config); caller should ``disable`` first.
+    """
+    src = disabled_path(pack_folder)
+    if not src.exists():
+        return False
+    dst = config_path(pack_folder)
+    if dst.exists():
+        raise FileExistsError(
+            f"refusing to enable: sync.json already exists at {dst}. "
+            "Run 'disable' first if you want to swap configs."
+        )
+    src.replace(dst)
+    return True
+
+
+def delete(pack_folder: Path) -> bool:
+    """Hard-delete sync.json (and any .disabled sibling). Used by tests
+    and as an escape hatch — the ``disable`` subcommand uses the rename
+    flow instead so re-enabling preserves the same instance_id."""
+    removed = False
+    for p in (config_path(pack_folder), disabled_path(pack_folder)):
+        if p.exists():
+            p.unlink()
+            removed = True
+    return removed
+
+
 # ---------------------------------------------------------------------------
 # minting
 # ---------------------------------------------------------------------------