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/infrastructure/src/best_practice/mod.rs

@@ -22,6 +22,7 @@
 pub mod arch_audit;
 pub mod code_quality;
 pub mod commit;
+pub mod explore;
 pub mod skills;
 
 use anyhow::Result;