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/explore.rs

@@ -0,0 +1,52 @@
+//! Mandatory explore phase — spawns parallel subagents to discover context
+//! before the main agent begins its turn.
+//!
+//! # Flow
+//!
+//! Before the main agent's LLM loop, [`ExploreService::explore`] dispatches
+//! at least 3 subagents in parallel (code-structure scan, symbol-index query,
+//! semantic-context search). Their findings are consolidated into a single
+//! system message that is prepended to the conversation.
+//!
+//! # Why mandatory
+//!
+//! Without structured exploration the main agent works from an empty context
+//! window. The explore phase guarantees that every turn starts with a compact
+//! snapshot of what the codebase contains and where relevant code lives.
+
+use anyhow::Result;
+use std::future::Future;
+use std::pin::Pin;
+
+/// The consolidated output of an explore phase — a set of system-level
+/// context messages injected before the main agent prompt.
+#[derive(Debug, Clone)]
+pub struct ExploreOutput {
+    /// One or more system messages summarising what the explore subagents
+    /// discovered. Prepended to the conversation by the turn service.
+    pub context_messages: Vec<String>,
+    /// Short human-readable summary of what was explored.
+    pub summary: String,
+}
+
+/// Service trait for the mandatory pre-turn exploration phase.
+///
+/// Implementors spawn ≥3 parallel subagents, each analysing a different
+/// aspect of the workspace, and return a consolidated summary.
+///
+/// # Object safety
+///
+/// This trait is `dyn`-safe — it returns `Pin<Box<dyn Future>>` so it can
+/// be stored as `Arc<dyn ExploreService>`.
+pub trait ExploreService: Send + Sync {
+    /// Run the explore phase.
+    ///
+    /// `query` — the user's current input phrase.
+    /// `workspace_root` — absolute path to the workspace root.
+    /// Returns structured context messages and a summary blob.
+    fn explore<'a>(
+        &'a self,
+        query: &'a str,
+        workspace_root: &'a str,
+    ) -> Pin<Box<dyn Future<Output = Result<ExploreOutput>> + Send + 'a>>;
+}