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/lib.rs

@@ -51,4 +51,7 @@ pub use cms::{
     settings_service::SettingsServiceImpl,
 };
 
-pub use agent::{AgentTurnService, ToolExecutor, turn_service::{AgentTurnServiceImpl, compact_messages_with_ai}};
+pub use agent::{
+    AgentTurnService, ExploreOutput, ExploreService, ToolExecutor,
+    turn_service::{AgentTurnServiceImpl, compact_messages_with_ai},
+};