fix: include step content in continuation prompt (closes #466)

- Add `content: string | null` to RoleStep type
- Resolve contentHash → text for the last step when building ThreadContext
- Update buildAgentPrompt to include <output> tag with step content
- Add 16k content quota with truncation
- Update tests

packages/workflow-util-agent/src/build-agent-prompt.ts

@@ -0,0 +1,64 @@
+import type { AgentContext } from "@uncaged/workflow-runtime";
+
+/** Max characters of step content to include in the prompt. */
+const CONTENT_QUOTA = 16_000;
+
+/** Builds the full agent prompt: system instructions plus summarized thread history. */
+export async function buildAgentPrompt(ctx: AgentContext): Promise<string> {
+  const lines: string[] = [];
+  lines.push(ctx.currentRole.systemPrompt);
+  lines.push("");
+  lines.push("## Task");
+  lines.push(ctx.start.content);
+
+  const { steps } = ctx;
+  if (steps.length === 0) {
+    return lines.join("\n");
+  }
+
+  if (steps.length === 1) {
+    const s = steps[0];
+    lines.push("");
+    lines.push(`## Step: ${s.role}`);
+    lines.push("");
+    lines.push(`Meta: ${JSON.stringify(s.meta)}`);
+    appendContent(lines, s.content);
+  } else {
+    lines.push("");
+    lines.push("## Previous Steps");
+    for (let i = 0; i < steps.length - 1; i++) {
+      const s = steps[i];
+      lines.push("");
+      lines.push(`### Step ${i + 1}: ${s.role}`);
+      lines.push(`Summary: ${JSON.stringify(s.meta)}`);
+    }
+    const last = steps[steps.length - 1];
+    lines.push("");
+    lines.push(`## Latest Step: ${last.role}`);
+    lines.push("");
+    lines.push(`Meta: ${JSON.stringify(last.meta)}`);
+    appendContent(lines, last.content);
+  }
+
+  lines.push("");
+  lines.push("## Tools");
+  lines.push(
+    `Use \`uncaged-workflow thread ${ctx.threadId}\` to read full details of any previous step.`,
+  );
+
+  return lines.join("\n");
+}
+
+function appendContent(lines: string[], content: string | null | undefined): void {
+  if (content === null || content === undefined || content.trim() === "") {
+    return;
+  }
+  const truncated =
+    content.length > CONTENT_QUOTA
+      ? `${content.slice(0, CONTENT_QUOTA)}\n... (truncated)`
+      : content;
+  lines.push("");
+  lines.push("<output>");
+  lines.push(truncated);
+  lines.push("</output>");
+}