refactor(shared): move parseModelString out of delegate-task to break cross-tool coupling

Move parseModelString into src/shared so callers can depend on a neutral module instead of reaching into delegate-task internals.

Cross-tool coupling violates module boundaries, and this keeps call-omo-agent plus runtime-fallback from importing through a sibling tool.

src/hooks/runtime-fallback/retry-model-payload.ts

@@ -1,4 +1,4 @@
-import { parseModelString } from "../../tools/delegate-task/model-string-parser"
+import { parseModelString } from "../../shared/model-string-parser"
 
 export function buildRetryModelPayload(
   model: string,