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

src/checks/todos.ts

@@ -124,6 +124,21 @@ const MAX_CANDIDATES = 500;
 const FALLBACK_CAP = 16;
 /** Candidate list embedded in the live prompt is truncated at this many. */
 const PROMPT_CAP = 40;
+/**
+ * Max characters of a candidate's code line embedded in the task prompt.
+ * Generated/bundled single lines can be hundreds of KB (e.g. minified assets
+ * sneaking past scope); embedding them wholesale balloons the task to
+ * megabytes and chokes the sub-agent. `path:line` plus a truncated prefix is
+ * enough to classify — the agent can read the file for full context.
+ */
+const CODE_DISPLAY_CAP = 160;
+
+/** Truncate a candidate's code line for prompt embedding. */
+function displayCode(code: string): string {
+	return code.length > CODE_DISPLAY_CAP
+		? `${code.slice(0, CODE_DISPLAY_CAP)}…`
+		: code;
+}
 
 /** Extract the declared function name from a header line, when present. */
 function headerName(line: string): string | undefined {
@@ -369,7 +384,7 @@ function renderFindings(
 		parts.push(`## ${title}`);
 		list.slice(0, FALLBACK_CAP).forEach((c, i) => {
 			parts.push(
-				`### ${i + 1}. ${relative(cwd, c.path)}:${c.line} — ${c.code} | snippet: ${c.snippet} | context: ${c.context || relative(cwd, c.path)}`,
+				`### ${i + 1}. ${relative(cwd, c.path)}:${c.line} — ${displayCode(c.code)} | snippet: ${c.snippet} | context: ${c.context || relative(cwd, c.path)}`,
 			);
 		});
 		if (list.length > FALLBACK_CAP) {
@@ -434,7 +449,7 @@ export async function buildTodosScanTask(
 
 	const candidateList = candidates
 		.slice(0, PROMPT_CAP)
-		.map((c) => ` - ${relative(cwd, c.path)}:${c.line} [${c.kind}] ${c.code}`);
+		.map((c) => ` - ${relative(cwd, c.path)}:${c.line} [${c.kind}] ${displayCode(c.code)}`);
 	if (candidates.length > PROMPT_CAP) {
 		candidateList.push(
 			` - … and ${candidates.length - PROMPT_CAP} more (truncated for brevity)`,