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.

README.md

@@ -48,13 +48,19 @@ python /path/to/cloud-sync.pyz init \
 
 That mints a fresh `instance_id` (ULID), writes `.cloud-sync/sync.json` (the opt-in marker) and `.cloud-sync/token` (mode 600). Subsequent launches sync automatically via the global hook.
 
-**Opting out:**
+**Opting out / pausing:**
 
 ```
 python /path/to/cloud-sync.pyz disable --pack-folder=/path/to/instance/minecraft
 ```
 
-Removes `sync.json` (cloud data untouched). Instance returns to no-op behavior.
+Renames `sync.json` → `sync.json.disabled`. Instance returns to no-op behavior (the opt-in gate only sees a literal `sync.json`). Re-enable later with:
+
+```
+python /path/to/cloud-sync.pyz enable  --pack-folder=/path/to/instance/minecraft
+```
+
+That rename-back preserves the original `instance_id` so snapshots continue against the same restic repo — no fresh ULID, no orphaned history. For a truly fresh start, `disable` then delete `sync.json.disabled` by hand before re-running `setup`.
 
 Player needs Python 3.10+ AND a Qt binding (`pip install PySide6`). Without Qt the pyz falls back to headless mode for status; the conflict + login dialogs are Qt-only — without them, conflict aborts the launch defensively and `setup` prompts for the token via stdin.
 
@@ -70,7 +76,8 @@ instance-sync pull / push [--pack-folder=PATH]
 
 instance-sync setup       [--url=URL] [--pack-folder=PATH]     # interactive
 instance-sync init        --url=URL [--token=ID:PASS] ...      # scripted
-instance-sync disable     [--pack-folder=PATH]
+instance-sync disable     [--pack-folder=PATH]                 # sync.json → sync.json.disabled
+instance-sync enable      [--pack-folder=PATH]                 # sync.json.disabled → sync.json
 ```
 
 `pull` and `push` don't take `--url` — it's loaded from `sync.json`. `--instance-label` overrides the UI display name (defaults to `$INST_NAME` from Prism, then `$INST_ID`, then the first 8 chars of `instance_id`).