feat: add two-layer thinking block validation (proactive + reactive) (#248)

- Add thinking-block-validator hook for proactive prevention before API calls
- Enhance session-recovery to include previous thinking content
- Fix hook registration to actually invoke the validator

Addresses extended thinking errors with Claude Opus/Sonnet 4.5 using tool calls.

Related: https://github.com/vercel/ai/issues/7729
Related: https://github.com/sst/opencode/issues/2599

src/hooks/index.ts

@@ -21,3 +21,4 @@ export { createKeywordDetectorHook } from "./keyword-detector";
 export { createNonInteractiveEnvHook } from "./non-interactive-env";
 export { createInteractiveBashSessionHook } from "./interactive-bash-session";
 export { createEmptyMessageSanitizerHook } from "./empty-message-sanitizer";
+export { createThinkingBlockValidatorHook } from "./thinking-block-validator";