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/todos.test.ts

@@ -205,6 +205,30 @@ describe("detectTodoStubs", () => {
 			loud.some((h) => h.path.endsWith("fetch.rs") && h.snippet === "todo!("),
 		).toBe(true);
 	});
+
+	it("never descends into build/deploy output directories", async () => {
+		// Generated bundles under framework build dirs must not feed the
+		// pre-scan: they dominate candidate counts with minified noise (the
+		// freno-dev failure flagged 119 of 124 candidates inside
+		// `.output`/`.vercel` bundles, ballooning the scan task to 2.5 MB).
+		for (const rel of [
+			join(".output", "public", "bundle.js"),
+			join(".vercel", "output", "static", "app.js"),
+			join(".netlify", "functions", "bundle.js"),
+		]) {
+			const full = join(dir, rel);
+			await mkdir(join(full, ".."), { recursive: true });
+			await writeFile(
+				full,
+				"// TODO: bundle placeholder\nfunction f(){ return 0; }\nthrow new Error('not implemented');\n",
+				"utf8",
+			);
+		}
+		const hits = await detectTodoStubs(dir);
+		expect(
+			hits.filter((h) => /(?:\.output|\.vercel|\.netlify)[/\\]/.test(h.path)),
+		).toHaveLength(0);
+	});
 });
 
 describe("todos check", () => {
@@ -334,4 +358,25 @@ describe("todos check", () => {
 		);
 		expect(task).toContain("| new: 0 | resolved: 4 |");
 	});
+
+	it("truncates giant single-line candidates so the task prompt stays bounded", async () => {
+		// A minified/generated single line can be hundreds of KB; embedding it
+		// wholesale ballooned the freno-dev task to 2.5 MB and choked the
+		// analysis agent. The task must carry a truncated prefix, never the
+		// full line.
+		const long = `// TODO: ${"x".repeat(400)}`;
+		await writeFile(
+			join(cwd, "huge.ts"),
+			`${long}\nexport function f() { return 0; }\n`,
+			"utf8",
+		);
+		const task = await buildTodosScanTask(cwd, {
+			cwd,
+			target: cwd,
+			fix: false,
+			rest: [],
+		});
+		expect(task).not.toContain("x".repeat(400));
+		expect(task).toContain("…");
+	});
 });