feat: add hooks to nudge agents toward codebase_search tool

SubagentStart hook injects guidance for Plan and Explore agents to
think deeply about questions and fire consecutive codebase_search
calls. UserPromptSubmit hook injects a tip when prompts contain
"Where", "Explain", or "Find" keywords.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>