feat(context-window): show current/max context usage per tab/model

Add a 'Context Window' sidebar view showing the live context occupancy
(latest request's input+output) against the model's maximum context
window, resolved dynamically from the models.dev catalog.

- core: models.dev catalog module (resolveContextLimit) with disk cache,
  TTL, stale-fallback + offline penalty memo; null for unknown models.
- api: GET /models/context-limit?provider=&modelId=.
- frontend: ContextWindowPanel + computeContextUsage helper; App resolves
  + caches the active model's max (anthropic/opencode-anthropic only);
  percent shown to 2 decimals; degrades to bare token count when max
  unknown.
- tests: core catalog (13), api route (3), frontend helper (6).

1 files changed, 5 insertions, 1 deletions

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 327b0a5..9d7133f 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -67,7 +67,11 @@ export {
 } from "./llm/debug-logger.js";
 export { createProvider } from "./llm/provider.js";
 // Models
-export { ModelRegistry } from "./models/index.js";
+export {
+	getModelsCatalog,
+	ModelRegistry,
+	resolveContextLimit,
+} from "./models/index.js";
 // Notifications (ntfy.sh)
 export * from "./notifications/index.js";
 export * from "./permission/index.js";