feat(tickets): création d'un assistant IA de ticket (backend + frontend)

Ajoute le chat assistant IA attaché à un ticket (#8).

Backend Rust :
- use cases OpenTicketAssistant/CloseTicketAssistant + tests
- politique d'outils par agent (domain/agent_tool_policy) et policy MCP
- store de contexte assistant + gabarit default_ticket_assistant.md + tests
- events TicketAssistantOpened/Closed
- commandes Tauri open_ticket_chat/close_ticket_chat et câblage state/lib/events

Frontend :
- gateway (ports, adapters ticket + mock, domain)
- hook useTicketAssistant + composant TicketAssistantPanel
- intégration dans TicketDetail

Tests verts : vitest tickets.test.tsx (23), cargo test application::ticket_assistant (1),
infrastructure::assistant_context_store (2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/src/orchestrator/mcp/policy.rs

@@ -0,0 +1,76 @@
+//! In-memory MCP tool policy registry.
+
+use std::collections::HashMap;
+use std::sync::RwLock;
+
+use domain::AgentToolPolicy;
+use domain::AgentToolPolicyStore;
+
+/// Stores per-requester MCP tool policies for live assistant sessions.
+#[derive(Default)]
+pub struct ToolPolicyRegistry {
+    policies: RwLock<HashMap<String, AgentToolPolicy>>,
+}
+
+impl ToolPolicyRegistry {
+    /// Builds an empty registry.
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Sets or replaces the policy for `requester`.
+    pub fn set(&self, requester: impl Into<String>, policy: AgentToolPolicy) {
+        self.policies
+            .write()
+            .unwrap()
+            .insert(requester.into(), policy);
+    }
+
+    /// Returns the policy for `requester`, if any.
+    #[must_use]
+    pub fn get(&self, requester: &str) -> Option<AgentToolPolicy> {
+        self.policies.read().unwrap().get(requester).cloned()
+    }
+
+    /// Clears the policy for `requester`.
+    pub fn clear(&self, requester: &str) {
+        self.policies.write().unwrap().remove(requester);
+    }
+}
+
+impl AgentToolPolicyStore for ToolPolicyRegistry {
+    fn set_policy(&self, requester: String, policy: AgentToolPolicy) {
+        self.set(requester, policy);
+    }
+
+    fn get_policy(&self, requester: &str) -> Option<AgentToolPolicy> {
+        self.get(requester)
+    }
+
+    fn clear_policy(&self, requester: &str) {
+        self.clear(requester);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use domain::IssueRef;
+
+    use super::*;
+
+    #[test]
+    fn set_get_clear_roundtrips_by_requester() {
+        let registry = ToolPolicyRegistry::new();
+        let policy = AgentToolPolicy::new(
+            vec!["idea_ticket_read".to_owned()],
+            Some("#7".parse::<IssueRef>().unwrap()),
+            true,
+        );
+        registry.set("assistant-1", policy.clone());
+        assert_eq!(registry.get("assistant-1"), Some(policy));
+        assert_eq!(registry.get("assistant-2"), None);
+        registry.clear("assistant-1");
+        assert_eq!(registry.get("assistant-1"), None);
+    }
+}