fix: detect token-limit errors in todo-continuation to prevent infinite loop (#2462)

handler.ts now catches ContextLengthError/prompt-too-long errors and
stops continuation instead of retrying with an even larger context.

91 tests pass, tsc clean.

Closes #2462

src/hooks/todo-continuation-enforcer/token-limit-detection.ts

@@ -0,0 +1,27 @@
+const TOKEN_LIMIT_ERROR_NAMES = new Set([
+  "contextlengtherror",
+])
+
+const TOKEN_LIMIT_KEYWORDS = [
+  "prompt is too long",
+  "is too long",
+  "context_length_exceeded",
+  "token limit",
+  "context length",
+  "too many tokens",
+]
+
+export function isTokenLimitError(error: { name?: string; message?: string } | undefined): boolean {
+  if (!error) return false
+
+  if (error.name && TOKEN_LIMIT_ERROR_NAMES.has(error.name.toLowerCase())) {
+    return true
+  }
+
+  if (error.message) {
+    const lower = error.message.toLowerCase()
+    return TOKEN_LIMIT_KEYWORDS.some((keyword) => lower.includes(keyword))
+  }
+
+  return false
+}