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, 23 insertions, 0 deletions

diff --git a/packages/cli/src/args.ts b/packages/cli/src/args.ts
index ecf6e2e..ac5dd4a 100644
--- a/packages/cli/src/args.ts
+++ b/packages/cli/src/args.ts
@@ -47,6 +47,7 @@ export type ParsedCommand =
 			readonly cwd?: string;
 			readonly reasoningEffort?: ReasoningEffort;
 	  }
+	| { readonly kind: "stop"; readonly server: string; readonly conversationId: string }
 	| { readonly kind: "help" }
 	| { readonly kind: "error"; readonly message: string };
 
@@ -131,6 +132,28 @@ export function parseArgs(argv: readonly string[], opts: ParseOpts): ParsedComma
 		return { kind: "compact", server, conversationId };
 	}
 
+	if (first === "stop") {
+		let server = opts.defaultServer;
+		let conversationId: string | undefined;
+		for (let i = 1; i < argv.length; i++) {
+			const arg = argv[i] as string;
+			if (arg === "--server") {
+				if (i + 1 >= argv.length) return { kind: "error", message: "--server requires a value" };
+				server = argv[++i] as string;
+			} else if (arg.startsWith("--")) {
+				return { kind: "error", message: `Unknown flag: ${arg}` };
+			} else if (conversationId !== undefined) {
+				return { kind: "error", message: `Unexpected argument for 'stop': ${arg}` };
+			} else {
+				conversationId = arg;
+			}
+		}
+		if (conversationId === undefined) {
+			return { kind: "error", message: "'stop' requires a conversation id" };
+		}
+		return { kind: "stop", server, conversationId };
+	}
+
 	if (first === "read") {
 		let server = opts.defaultServer;
 		let conversationId: string | undefined;