feat(R03): unify model routing and centralise the provider catalogue

EPIC R03 (Team Duy) — Model Providers & Routing. All six tasks done.

R03-T02 — Provider catalogue
  domain/models/provider_descriptor.py     ProviderDescriptor (frozen), WireProtocol, AuthKind
  infrastructure/providers/provider_registry.py
                                           thread-safe registry: id/alias lookup, dynamic
                                           lookup by model id, adapter selection by protocol
  providers/factory.py                     drops its own _REGISTRY table and delegates to the
                                           registry, still raising ProviderError for callers

R03-T03 — RoutingApplicationService (pure Python, 4 modes)
  application/model_routing/routing_models.py
                                           RoutingMode (off/auto/manual/fallback),
                                           RoutingRequest (immutable snapshot), RouteEvaluation,
                                           RoutingOutcome
  application/model_routing/routing_application_service.py
                                           the single decision flow, reached through two narrow
                                           ports plus a caller-supplied confirm callback, so no
                                           Qt import is needed
  application/model_routing/core_routing_adapter.py
                                           binds the ports to core/routing and AppContext

  Fallback is a new resilience mode: keep the selected model while it can serve the turn,
  re-route only when it cannot. Wired end to end through config.py, state.py,
  ui/routing_toggle.py and i18n.py (EN/JA/VI).

R03-T04 / T05 — Remove the duplicated routing flow
  ui/chat_panel.py (#L638), ui/co4e_tab.py, ui/folder_tab.py each drop ~35 lines of copied
  logic and call the shared service; the widgets now only build a RoutingRequest, host the
  Manual-mode modal and render the outcome.

R03-T06 — Token usage as an event
  infrastructure/telemetry/usage_sink.py   UsageEvent + UsageEventSink protocol, with tracker,
                                           in-memory and composite sinks
  providers/openai_compat.py, providers/anthropic.py
                                           publish a UsageEvent instead of writing to the
                                           usage tracker themselves
  core/usage_tracker.py                    adds current_context() so a sink can borrow and
                                           restore a thread's attribution

R03-T01 — Contract tests
  tests/contracts/test_providers.py parametrises over every provider in the registry: chat()
  signature, canonical assistant message, normalised tool calls, response closed, tool schema
  translation, ProviderError, list_models/test_connection, one UsageEvent per turn.

Test infrastructure fix (required to verify any of the above): tests/conftest.py used to put
the repository's PARENT directory on sys.path, so `import cowork_local.*` resolved against
whichever sibling folder happened to carry that name — on a dev machine, an unrelated older
checkout. The suite reported green while exercising different code. The conftest now binds
this checkout to the cowork_local name in sys.modules.

Verification
  pytest tests/                    236 passed in ~1.8s (102 before this change)
  scripts/check_imports.py         PASS, 0 forbidden imports in domain/ and application/
  new production files             largest is 288 lines, all under the 400 LOC ceiling
  new tests                        134 (50 contract, 70 unit, 14 integration), all offline

scripts/run_quality_gate.py does not exist yet (R10-T02), so DoD item 7 was covered by
check_imports.py plus the full suite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

i18n.py

@@ -583,10 +583,13 @@ STRINGS: Dict[str, Dict[str, str]] = {
     "routing.mode_off": {"en": "Off", "ja": "オフ", "vi": "Tắt"},
     "routing.mode_auto": {"en": "Auto", "ja": "自動", "vi": "Tự động"},
     "routing.mode_manual": {"en": "Manual", "ja": "手動", "vi": "Thủ công"},
+    # Fallback (R03-T03): resilience mode -- never switches for a better
+    # score, only to rescue a selected model that cannot serve the turn.
+    "routing.mode_fallback": {"en": "Fallback", "ja": "フォールバック", "vi": "Dự phòng"},
     "routing.toggle_tooltip": {
-        "en": "Auto model routing for this chat.\nOff: always use the selected model.\nAuto: silently switch to the best-fit model.\nManual: ask before switching.",
-        "ja": "このチャットの自動モデルルーティング。\nオフ: 選択したモデルを常に使用。\n自動: 最適なモデルへ自動切替。\n手動: 切替前に確認。",
-        "vi": "Tự động định tuyến model cho khung chat này.\nTắt: luôn dùng model đã chọn.\nTự động: tự chuyển sang model phù hợp nhất.\nThủ công: hỏi xác nhận trước khi chuyển.",
+        "en": "Auto model routing for this chat.\nOff: always use the selected model.\nAuto: silently switch to the best-fit model.\nManual: ask before switching.\nFallback: keep the selected model, switch only if it is unavailable.",
+        "ja": "このチャットの自動モデルルーティング。\nオフ: 選択したモデルを常に使用。\n自動: 最適なモデルへ自動切替。\n手動: 切替前に確認。\nフォールバック: 選択モデルを維持し、利用できない場合のみ切替。",
+        "vi": "Tự động định tuyến model cho khung chat này.\nTắt: luôn dùng model đã chọn.\nTự động: tự chuyển sang model phù hợp nhất.\nThủ công: hỏi xác nhận trước khi chuyển.\nDự phòng: giữ model đã chọn, chỉ chuyển khi model đó không dùng được.",
     },
     "routing.confirm_title": {
         "en": "Switch model?", "ja": "モデルを切り替えますか？", "vi": "Chuyển model?",