feat: stop generation mid-turn (POST /conversations/:id/stop)

Add stopTurn to the orchestrator: aborts the in-flight turn's
AbortController without changing conversation status. The turn
seals normally (finishReason: 'aborted'), partial messages are
persisted, and the conversation transitions active → idle via the
normal settle path.

Distinct from closeConversation which marks the conversation closed.

- POST /conversations/:id/stop endpoint
- dispatch stop <id> CLI command
- FE handoff: frontend-stop-generation-handoff.md

1 files changed, 17 insertions, 0 deletions

diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index b46ecc1..bc9e78b 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -218,6 +218,14 @@ export interface SessionOrchestrator {
 	 * Idempotent — closing an idle/unknown conversation just emits the hook.
 	 */
 	closeConversation(conversationId: string): { readonly abortedTurn: boolean };
+	/**
+	 * Stop an in-flight generation WITHOUT closing the conversation. Aborts
+	 * the turn's AbortController — the kernel finishes with
+	 * `finishReason: "aborted"`, partial messages are persisted, and the turn
+	 * seals normally (status transitions active → idle via the normal settle
+	 * path). Idempotent — stopping an idle/unknown conversation is a no-op.
+	 */
+	stopTurn(conversationId: string): { readonly abortedTurn: boolean };
 	handleMessage(input: {
 		conversationId: string;
 		text: string;
@@ -567,6 +575,15 @@ export function createSessionOrchestrator(
 			return { abortedTurn };
 		},
 
+		stopTurn(conversationId) {
+			const turn = activeTurns.get(conversationId);
+			const abortedTurn = turn !== undefined;
+			if (turn !== undefined) {
+				turn.controller.abort();
+			}
+			return { abortedTurn };
+		},
+
 		async handleMessage({ conversationId, text, onEvent, modelName, cwd, reasoningEffort }) {
 			const turnInput: StartTurnInput = {
 				conversationId,