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>

config.py

@@ -555,19 +555,26 @@ class AppConfig:
             d["surface_modes"].setdefault(surface, "")
         return d
 
-    def routing_mode_for(self, surface: str) -> str:
-        """Effective Off/Auto/Manual mode for a chat surface.
+    # The routing modes a surface may be in. "fallback" joined the set in
+    # R03-T03 (keep the selected model; re-route only when it cannot serve the
+    # turn) — see application/model_routing/routing_models.py::RoutingMode,
+    # which is the authority on what each mode means.
+    ROUTING_MODES = ("off", "auto", "manual", "fallback")
 
-        A per-surface override ("auto"/"manual"/"off") wins; an empty override
-        falls back to the global ``switch_mode``."""
+    def routing_mode_for(self, surface: str) -> str:
+        """Effective Off/Auto/Manual/Fallback mode for a chat surface.
+
+        A per-surface override wins; an empty override falls back to the global
+        ``switch_mode``. Anything unrecognised degrades to "off" so routing
+        stays opt-in even with a hand-edited config."""
         routing = self.routing
         override = (routing.get("surface_modes", {}) or {}).get(surface, "")
         mode = override or routing.get("switch_mode", "off")
-        return mode if mode in ("off", "auto", "manual") else "off"
+        return mode if mode in self.ROUTING_MODES else "off"
 
     def set_routing_mode_for(self, surface: str, mode: str) -> None:
-        """Persist a chat surface's Off/Auto/Manual toggle selection."""
-        mode = mode if mode in ("off", "auto", "manual") else "off"
+        """Persist a chat surface's routing toggle selection."""
+        mode = mode if mode in self.ROUTING_MODES else "off"
         self.routing.setdefault("surface_modes", {})[surface] = mode
         self.save()