feat(writes): enable write actions by default (opt-out)

Invert the global write toggle so writes are ENABLED by default and the
user opts OUT, while keeping all other safety intact (per-call confirm=true
dry-run preview and the on-disk audit log are unchanged).

- writes.py: use a `writes_disabled` flag file instead of `writes_enabled`;
  `writes_enabled()` returns True unless the flag exists (env var still
  honored either way); `set_writes(False)` creates the flag, `set_writes(True)`
  removes it. Update user-facing notes.
- tests: default is now ENABLED; adjust toggle/disabled tests (set disabled
  explicitly via set_writes(False) or FAMILIAR_ENABLE_WRITES=0).
- README.md / CLAUDE.md: document writes are on by default, opt out via
  `familiar writes disable`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_contacts_fittings.py

@@ -70,7 +70,7 @@ def test_get_fittings(mock_esi, fake_auth):
 
 def test_create_fitting_disabled_raises(mock_esi, fake_auth, tmp_path, monkeypatch):
     monkeypatch.setenv("FAMILIAR_DATA_DIR", str(tmp_path))
-    monkeypatch.delenv("FAMILIAR_ENABLE_WRITES", raising=False)
+    monkeypatch.setenv("FAMILIAR_ENABLE_WRITES", "0")
     mock_esi(lambda r: httpx.Response(201, json={"fitting_id": 5}))
     with pytest.raises(writes.WriteDisabled):
         asyncio.run(fittings.create_fitting(99, {"name": "x"}, confirm=True))