feat(slack): add configurable stream modes

src/slack/stream-mode.ts

@@ -0,0 +1,53 @@
+export type SlackStreamMode = "replace" | "status_final" | "append";
+
+const DEFAULT_STREAM_MODE: SlackStreamMode = "replace";
+
+export function resolveSlackStreamMode(raw: unknown): SlackStreamMode {
+  if (typeof raw !== "string") {
+    return DEFAULT_STREAM_MODE;
+  }
+  const normalized = raw.trim().toLowerCase();
+  if (normalized === "replace" || normalized === "status_final" || normalized === "append") {
+    return normalized;
+  }
+  return DEFAULT_STREAM_MODE;
+}
+
+export function applyAppendOnlyStreamUpdate(params: {
+  incoming: string;
+  rendered: string;
+  source: string;
+}): { rendered: string; source: string; changed: boolean } {
+  const incoming = params.incoming.trimEnd();
+  if (!incoming) {
+    return { rendered: params.rendered, source: params.source, changed: false };
+  }
+  if (!params.rendered) {
+    return { rendered: incoming, source: incoming, changed: true };
+  }
+  if (incoming === params.source) {
+    return { rendered: params.rendered, source: params.source, changed: false };
+  }
+
+  // Typical model partials are cumulative prefixes.
+  if (incoming.startsWith(params.source) || incoming.startsWith(params.rendered)) {
+    return { rendered: incoming, source: incoming, changed: incoming !== params.rendered };
+  }
+
+  // Ignore regressive shorter variants of the same stream.
+  if (params.source.startsWith(incoming)) {
+    return { rendered: params.rendered, source: params.source, changed: false };
+  }
+
+  const separator = params.rendered.endsWith("\n") ? "" : "\n";
+  return {
+    rendered: `${params.rendered}${separator}${incoming}`,
+    source: incoming,
+    changed: true,
+  };
+}
+
+export function buildStatusFinalPreviewText(updateCount: number): string {
+  const dots = ".".repeat((Math.max(1, updateCount) % 3) + 1);
+  return `Status: thinking${dots}`;
+}