feat: implement mandatory explore phase with parallel subagents
- Added `ExploreService` trait and `ExploreServiceImpl` struct to handle the exploration of codebase context before agent turns. - Implemented three parallel subagents: Code Structure, Symbol Index, and Semantic Context, each with specific directives. - Integrated the explore phase into the agent turn process, ensuring that each turn starts with a consolidated context message. - Enhanced `spawn_agent_turn` function to include explore service wiring and context preparation.
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