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/tools/registry.rs

@@ -44,6 +44,7 @@ pub fn all_tools() -> Vec<Box<dyn super::Tool>> {
         Box::new(super::web_search::WebSearch),
         Box::new(super::semantic_search::SemanticSearch),
         Box::new(super::semantic_search::RebuildIndex),
+        Box::new(super::semantic_search::ListSymbols),
         Box::new(super::parallel_delegate::ParallelDelegate),
         // ── Best-practice tools (built-in) ─────────────────────────
         Box::new(super::best_practice::BestPractice),