fix: todos scan task ballooned to 2.5MB and analysis produced no output

The todos pre-scan walked .output/ (Nitro) and .vercel/ (Vercel) build
dirs, flagging 119 of 124 candidates inside minified bundles (single
lines up to 162KB). buildTodosScanTask embedded full candidate lines in
the task prompt, producing a 2.5MB prompt on freno-dev; the analysis
agent settled with ok:true + empty text + no findings.md, verify failed,
and resume re-ran the same oversized prompt and failed identically.

- scope: exclude .output/.vercel/.netlify (shared by all checks)
- todos: truncate candidate code at 160 chars in the prompt + fallback
- agent-runner: a session settling with no text and no observed message/
  tool events now fails the run loudly instead of reporting ok:true
- agent prompts: add the three dirs to each skip list
- tests: excluded-dir scan, prompt truncation, emptySessionError cases

tests/agent-runner.test.ts

@@ -10,12 +10,13 @@
 import { describe, expect, it } from "bun:test";
 import {
 	applySessionEvent,
+	emptySessionError,
 	type SessionEventAccumulator,
 } from "../src/agent-runner.js";
 import type { AgentSessionEvent } from "@earendil-works/pi-coding-agent";
 
 function fresh(): SessionEventAccumulator {
-	return { text: "" };
+	return { text: "", sawMessage: false };
 }
 
 /** Build a typed-as-unknown event so malformed shapes compile in tests. */
@@ -134,4 +135,54 @@ describe("applySessionEvent", () => {
 		expect(forwarded).toEqual([]);
 		expect(acc.text).toBe("x");
 	});
+
+	it("marks sawMessage when any message or tool event is observed", () => {
+		const fromUpdate = fresh();
+		applySessionEvent(fromUpdate, event({ type: "message_update" }));
+		expect(fromUpdate.sawMessage).toBe(true);
+
+		const fromEnd = fresh();
+		applySessionEvent(fromEnd, event({ type: "message_end", message: null }));
+		expect(fromEnd.sawMessage).toBe(true);
+
+		const fromTool = fresh();
+		applySessionEvent(fromTool, event({ type: "tool_execution_start" }));
+		expect(fromTool.sawMessage).toBe(true);
+	});
+});
+
+describe("emptySessionError", () => {
+	it("fails a session that settled with no text and no observed events", () => {
+		expect(emptySessionError({ text: "", sawMessage: false })).toContain(
+			"no output",
+		);
+	});
+
+	it("fails an empty session whose final message reported stopReason error", () => {
+		expect(
+			emptySessionError({ text: "", sawMessage: true, stopReason: "error" }),
+		).toBe("sub-agent session ended in error with no output.");
+	});
+
+	it("surfaces a recorded errorMessage regardless of text", () => {
+		expect(
+			emptySessionError({
+				text: "partial output",
+				sawMessage: true,
+				errorMessage: "upstream 529",
+			}),
+		).toBe("upstream 529");
+	});
+
+	it("accepts an empty-text session that demonstrably ran (tool activity)", () => {
+		expect(
+			emptySessionError({ text: "", sawMessage: true, stopReason: "end_turn" }),
+		).toBeUndefined();
+	});
+
+	it("accepts any session with text", () => {
+		expect(
+			emptySessionError({ text: "report", sawMessage: false }),
+		).toBeUndefined();
+	});
 });