feat(agent): add reasoning mode types and thinking extraction

Add ReasoningMode type (off/on/stream) to AgentOptions and related
config types. Add extractThinking() for extracting thinking content
blocks from LLM responses, mirroring the existing extractText() pattern.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/session/types.ts

@@ -4,6 +4,8 @@ export type SessionMeta = {
   provider?: string;
   model?: string;
   thinkingLevel?: string;
+  /** Reasoning mode: off, on, stream */
+  reasoningMode?: string;
   /** Context window token 数 */
   contextWindowTokens?: number;
 };