fix(agents): enforcer l'invariant « 1 session vivante par agent » (singleton)

Un agent ne peut tourner que dans une seule cellule à la fois. La garde dans
LaunchAgent refuse le spawn si l'agent est déjà vivant dans un autre node
(AGENT_ALREADY_RUNNING) ; idempotent sur le même node ; le chemin resume
(agent mort) reste inchangé. Le node_id est désormais plombé jusqu'au use case.

Corrige le reset asymétrique d'une cellule au changement d'onglet : deux leaves
partageant le même agent id rendaient session_for_agent/is_agent_live/stop_agent
ambigus (cible arbitraire). Le churn reset/reattach déclenchait aussi les accents
mélangés (FIFO intact, non touché).

- snapshot agentWasRunning calculé par node (is_node_live) et non par agent
- commande list_live_agents + live_agents()/node_for_agent()/is_node_live()
- UI : dropdown grise les agents déjà placés ailleurs ; 2e cellule en doublon
  affiche « disponible » au lieu d'une relance fantôme

Tests : cargo test (application + app-tauri) vert ; tsc + vitest vert.

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

crates/app-tauri/src/commands.rs

@@ -23,7 +23,8 @@ use application::{
 use domain::ports::PtyHandle;
 
 use crate::dto::{
-    parse_agent_id, parse_close_terminal, parse_delete_profile, parse_layout_id, parse_profile_id,
+    parse_agent_id, parse_close_terminal, parse_delete_profile, parse_layout_id, parse_node_id,
+    parse_profile_id, LiveAgentListDto,
     parse_project_id, parse_session_id, parse_template_id, AgentDriftListDto, AgentDto,
     AgentListDto, ConfigureProfilesRequestDto, CreateAgentFromTemplateRequestDto,
     CreateAgentRequestDto, CreateLayoutRequestDto, CreateLayoutResultDto, CreateProjectRequestDto,
@@ -689,6 +690,30 @@ pub async fn list_agents(
         .map_err(ErrorDto::from)
 }
 
+/// `list_live_agents` — list the agents that currently own a live PTY session
+/// and the cell hosting each, so the UI can disable an agent already running in
+/// another cell (the "one live session per agent" invariant).
+///
+/// `project_id` is accepted for API symmetry and future per-project scoping; the
+/// session registry is process-wide today, so the full live set is returned (a
+/// project's agent ids are disjoint from other projects' by construction, so the
+/// frontend can filter by the agents it knows).
+///
+/// # Errors
+/// Returns an [`ErrorDto`] (`INVALID` for a malformed project id).
+#[tauri::command]
+pub fn list_live_agents(
+    project_id: String,
+    state: State<'_, AppState>,
+) -> Result<LiveAgentListDto, ErrorDto> {
+    // Validate the id shape for a consistent contract, even though the registry
+    // is not project-scoped yet.
+    let _ = parse_project_id(&project_id)?;
+    Ok(LiveAgentListDto::from_pairs(
+        state.terminal_sessions.live_agents(),
+    ))
+}
+
 /// `read_agent_context` — read an agent's Markdown context.
 ///
 /// # Errors
@@ -803,6 +828,10 @@ pub async fn launch_agent(
 ) -> Result<TerminalSessionDto, ErrorDto> {
     let project = resolve_project(&request.project_id, &state).await?;
     let agent_id = parse_agent_id(&request.agent_id)?;
+    // The hosting cell drives the singleton-invariant guard. Parse it when the
+    // frontend supplies one; absent ⇒ `None` (a fresh node is minted, and an
+    // already-live agent is refused).
+    let node_id = request.node_id.as_deref().map(parse_node_id).transpose()?;
 
     let output = state
         .launch_agent
@@ -811,10 +840,9 @@ pub async fn launch_agent(
             agent_id,
             rows: request.rows,
             cols: request.cols,
-            node_id: None,
+            node_id,
             // Resume id is a property of the hosting cell; the frontend passes the
-            // leaf's current conversation id here. `None` until the launch_agent
-            // command is made layout-aware (T4 wiring follow-up).
+            // leaf's current conversation id here.
             conversation_id: request.conversation_id.clone(),
         })
         .await

crates/app-tauri/src/dto.rs

@@ -944,6 +944,13 @@ pub struct LaunchAgentRequestDto {
     /// launch may assign a new id when the profile supports it).
     #[serde(default)]
     pub conversation_id: Option<String>,
+    /// The layout leaf (node) hosting this launch. Enforces the "one live session
+    /// per agent" invariant: a launch into a node different from where the agent
+    /// is already running is refused (`AGENT_ALREADY_RUNNING`); the same node is
+    /// idempotent. Absent ⇒ a fresh node is minted (and any already-live agent is
+    /// refused).
+    #[serde(default)]
+    pub node_id: Option<String>,
 }
 
 impl From<LaunchAgentOutput> for TerminalSessionDto {
@@ -997,6 +1004,40 @@ impl From<InspectConversationOutput> for ConversationDetailsDto {
     }
 }
 
+/// One currently-live agent and the cell hosting it, for `list_live_agents`.
+///
+/// Lets the UI disable an agent already running in another cell (it cannot be
+/// launched a second time — the "one live session per agent" invariant).
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct LiveAgentDto {
+    /// The live agent's id (UUID string).
+    pub agent_id: String,
+    /// The hosting layout leaf's node id (UUID string).
+    pub node_id: String,
+}
+
+/// Response DTO for `list_live_agents` (transparent array on the wire).
+#[derive(Debug, Clone, Serialize)]
+#[serde(transparent)]
+pub struct LiveAgentListDto(pub Vec<LiveAgentDto>);
+
+impl LiveAgentListDto {
+    /// Builds the wire list from the registry's `(AgentId, NodeId)` pairs.
+    #[must_use]
+    pub fn from_pairs(pairs: Vec<(AgentId, NodeId)>) -> Self {
+        Self(
+            pairs
+                .into_iter()
+                .map(|(agent_id, node_id)| LiveAgentDto {
+                    agent_id: agent_id.to_string(),
+                    node_id: node_id.to_string(),
+                })
+                .collect(),
+        )
+    }
+}
+
 /// Parses an agent-id string (UUID) coming from the frontend.
 ///
 /// # Errors

crates/app-tauri/src/lib.rs

@@ -113,6 +113,7 @@ pub fn run() {
             commands::configure_profiles,
             commands::create_agent,
             commands::list_agents,
+            commands::list_live_agents,
             commands::read_agent_context,
             commands::update_agent_context,
             commands::delete_agent,