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.

apps/application/src/agent/mod.rs

@@ -22,4 +22,8 @@ pub trait AgentTurnService: Send + Sync {
     ) -> impl Future<Output = Result<()>> + Send;
 }
 
+pub mod explore;
 pub mod turn_service;
+
+pub use explore::{ExploreOutput, ExploreService};
+pub use turn_service::{compact_messages_with_ai, AgentTurnServiceImpl};