fix(runtime): stop/restart d'agent visible robuste — kill réel, état stopped, relance froide (#196, QA verte)

Solidifie le cycle stop/restart d'un agent visible en cellule:

- terminal/registry: kill effectif de la TUI native et de la CLI custom, arrêt idempotent du live runtime (PTY + structuré), distinction nette « agent stoppé » vs « pas de session ».

- workstate + lifecycle: projection d'un état runtime explicite « stopped » (AgentRuntimeState: running/stopping/stopped/failedStart), réconcilié avec le live state; traitement des courses « agent not found ».

- relance froide via freshStart: repart de zéro sans réattacher une session morte et recharge la configuration courante de l'agent (y compris changement de modèle).

- contrat projeté côté adapters frontend (ports/domain/normalization/mock) pour la consommation UI future (#197/#198), sans surface UI ici.

Tests QA verts: workstate_actions, orchestrator_service stop_agent, agent_lifecycle (reopen/fresh_start), workstate stopped runtime state, vitest adapter stop_live_agent.

crates/application/src/agent/lifecycle.rs

@@ -963,6 +963,7 @@ impl ChangeAgentProfile {
                 mcp_runtime: None,
                 allow_structured_alongside_pty: false,
                 require_structured: false,
+                fresh_start: false,
             })
             .await?;
         Ok(Some(output.session))
@@ -1143,6 +1144,9 @@ pub struct LaunchAgentInput {
     ///
     /// Historical callers leave this `false`, preserving the PTY launch contract.
     pub require_structured: bool,
+    /// Cold restart request: ignore the cell's previous conversation id and create
+    /// a new provider/CLI conversation for this activation.
+    pub fresh_start: bool,
 }
 
 /// OS/runtime facts injected by the composition root (`app-tauri`) to materialise
@@ -1863,6 +1867,11 @@ impl LaunchAgent {
                 self.structured_routing_mode,
                 StructuredRoutingMode::RequireStructured
             );
+        let launch_conversation_id = if input.fresh_start {
+            None
+        } else {
+            input.conversation_id.clone()
+        };
 
         // 1b. Enforce the "one live session per agent" invariant (decision: an
         //     agent is a singleton that runs in a single cell at a time). This
@@ -1908,7 +1917,7 @@ impl LaunchAgent {
                     self.pty.kill(&handle).await?;
                 }
             } else {
-                match reattach_decision(input.node_id, host_node, input.conversation_id.as_deref())
+                match reattach_decision(input.node_id, host_node, launch_conversation_id.as_deref())
                 {
                     ReattachDecision::Rebind { node_id } => {
                         if let Some(session) = self.sessions.rebind_agent_node_in_project(
@@ -1971,7 +1980,7 @@ impl LaunchAgent {
                         reattach_decision(
                             input.node_id,
                             host_node,
-                            input.conversation_id.as_deref(),
+                            launch_conversation_id.as_deref(),
                         )
                     };
                     let node_id = match decision {
@@ -2061,7 +2070,11 @@ impl LaunchAgent {
         //     the cell's current `conversation_id`. Any id this launch *assigns* is
         //     returned in the output so the caller persists it on the leaf.
         let (session_plan, assigned_conversation_id) = self
-            .resolve_session_plan(&profile, input.conversation_id.clone(), &input.project.root)
+            .resolve_session_plan(
+                &profile,
+                launch_conversation_id.clone(),
+                &input.project.root,
+            )
             .await;
         let mut spec =
             self.runtime
@@ -2076,7 +2089,7 @@ impl LaunchAgent {
                 &manifest,
                 &agent,
                 content.clone(),
-                input.conversation_id.as_deref(),
+                launch_conversation_id.as_deref(),
             )
             .await?;
         // Best-effort contextual embedder suggestion (LOT C3, §14.5.5): the agent
@@ -2186,7 +2199,7 @@ impl LaunchAgent {
                 //    pure partagée avec `resolve_conversation` (aucun couplage à
                 //    l'orchestrateur). C'est cet id — et **non** l'id de session moteur —
                 //    qui retrouve log + handoff au (re)lancement (P7) et survit au swap.
-                let pair_conversation_id = input.conversation_id.clone().unwrap_or_else(|| {
+                let pair_conversation_id = launch_conversation_id.clone().unwrap_or_else(|| {
                     ConversationId::for_project_pair(
                         input.project.id,
                         ConversationParty::User,