fix(shared): extract shared context limit resolver to eliminate monitor/truncator drift

- New context-limit-resolver.ts with resolveActualContextLimit() shared helper
- Anthropic provider detection now uses .includes('anthropic') instead of hard-coded IDs
- Both context-window-monitor and dynamic-truncator use the shared resolver
- Added missing test cases: Anthropic+1M disabled+cached limit, non-Anthropic without cache

src/shared/index.ts

@@ -45,6 +45,7 @@ export type {
 export * from "./model-availability"
 export * from "./fallback-model-availability"
 export * from "./connected-providers-cache"
+export * from "./context-limit-resolver"
 export * from "./session-utils"
 export * from "./tmux"
 export * from "./model-suggestion-retry"