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/assistant/default_ticket_assistant.md

@@ -0,0 +1,5 @@
+# Ticket Assistant
+
+You are IdeA's ticket editing assistant. Help improve the bound ticket only.
+Read the project context when needed, propose concise changes, and use ticket
+tools only for the ticket explicitly injected below.

crates/infrastructure/src/assistant/mod.rs

@@ -0,0 +1,76 @@
+//! Ticket assistant infrastructure adapters.
+
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use domain::{
+    AssistantContextError, AssistantContextProvider, FileSystem, FsError, Issue, MarkdownDoc,
+    PreparedContext, Project, RemotePath,
+};
+
+const ASSISTANT_DIR: &str = "assistant";
+const TICKET_ASSISTANT_FILE: &str = "ticket-assistant.md";
+const DEFAULT_TICKET_ASSISTANT_CONTEXT: &str = include_str!("default_ticket_assistant.md");
+
+/// File-backed provider for the IdeA-owned ticket assistant context.
+#[derive(Clone)]
+pub struct FsAssistantContextStore {
+    fs: Arc<dyn FileSystem>,
+    app_data_dir: String,
+}
+
+impl FsAssistantContextStore {
+    /// Builds the store from an injected filesystem and app-data directory.
+    #[must_use]
+    pub fn new(fs: Arc<dyn FileSystem>, app_data_dir: impl Into<String>) -> Self {
+        Self {
+            fs,
+            app_data_dir: app_data_dir.into(),
+        }
+    }
+
+    fn join(&self, rel: &str) -> String {
+        let base = self.app_data_dir.trim_end_matches(['/', '\\']);
+        format!("{base}/{rel}")
+    }
+
+    fn context_path(&self) -> RemotePath {
+        RemotePath::new(self.join(&format!("{ASSISTANT_DIR}/{TICKET_ASSISTANT_FILE}")))
+    }
+
+    async fn read_base_context(&self) -> Result<String, AssistantContextError> {
+        let path = self.context_path();
+        match self.fs.read(&path).await {
+            Ok(bytes) => {
+                String::from_utf8(bytes).map_err(|e| AssistantContextError::Store(e.to_string()))
+            }
+            Err(FsError::NotFound(_)) => Ok(DEFAULT_TICKET_ASSISTANT_CONTEXT.to_owned()),
+            Err(e) => Err(AssistantContextError::Store(e.to_string())),
+        }
+    }
+}
+
+#[async_trait]
+impl AssistantContextProvider for FsAssistantContextStore {
+    async fn prepare_ticket_assistant_context(
+        &self,
+        project: &Project,
+        issue: &Issue,
+    ) -> Result<PreparedContext, AssistantContextError> {
+        let mut content = self.read_base_context().await?;
+        content.push_str("\n\n## Bound Ticket\n\n");
+        content.push_str(&format!("- Ref: {}\n", issue.reference()));
+        content.push_str(&format!("- Title: {}\n", issue.title));
+        content.push_str("\n### Description\n\n");
+        content.push_str(issue.description.as_str());
+        content.push_str("\n\n### Carnet\n\n");
+        content.push_str(issue.carnet.as_str());
+        content.push('\n');
+
+        Ok(PreparedContext {
+            content: MarkdownDoc::new(content),
+            relative_path: TICKET_ASSISTANT_FILE.to_owned(),
+            project_root: project.root.as_str().to_owned(),
+        })
+    }
+}