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.

tests/test_cli.py

@@ -99,6 +99,12 @@ def test_disable_subcommand_parses() -> None:
     assert str(ns.pack_folder) == "/tmp/x"
 
 
+def test_enable_subcommand_parses() -> None:
+    cmd, ns = parse(["enable", "--pack-folder=/tmp/x"])
+    assert cmd == "enable"
+    assert str(ns.pack_folder) == "/tmp/x"
+
+
 def test_missing_subcommand_exits() -> None:
     with pytest.raises(SystemExit):
         parse([])