refactor(shared): improve model resolver with 3-step resolution

Implement resolveModelWithFallback() that tries: 1) user override, 2) fuzzy match from requirements chain against available models, 3) system default. Export new model utilities from shared index.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/index.ts

@@ -26,4 +26,6 @@ export * from "./session-cursor"
 export * from "./shell-env"
 export * from "./system-directive"
 export * from "./agent-tool-restrictions"
+export * from "./model-requirements"
 export * from "./model-resolver"
+export * from "./model-availability"