feat: add pr list (task 0008)

Implements `pr list` with the two policies later PR slices reuse:
client-side filtering with the filtered-set count line (ADR 0005) and
the official-first reviewDecision via parallel per-PR review fetches
(ADR 0006), extracted into src/review.ts.

API-supported flags map to their params (--state, --author→poster,
--label name→ID, --label-id, --sort, --limit, --fields); --assignee,
--base, --head, and --draft filter in-process after full pagination,
with the count line's total taken from the filtered set. --search is
refused with a redirect to `search prs`.

Adds a boolText field extractor and a shared parsePositiveInt helper,
the latter also adopted by issue list's --limit parsing.

src/flags.ts

@@ -71,6 +71,27 @@ export function parseEnumFlag<T extends string>(
   return value as T;
 }
 
+/**
+ * Parse a flag's value as a positive integer, rejecting a bare switch or a
+ * non-integer with a uniform `VALIDATION_ERROR`. `label` names the flag in the
+ * message (e.g. "--limit", "--label-id"). Shared by every flag that takes one.
+ */
+export function parsePositiveInt(
+  value: string | true,
+  label: string,
+  suggestions: string[] = [],
+): number {
+  const parsed = Number(value);
+  if (value === true || !Number.isInteger(parsed) || parsed < 1) {
+    throw axiError(
+      `Invalid ${label} value: ${String(value)} (expected a positive integer)`,
+      "VALIDATION_ERROR",
+      suggestions,
+    );
+  }
+  return parsed;
+}
+
 /** Split "--flag=value" into name and inline value; "--flag" has none. */
 export function splitFlag(arg: string): SplitFlag {
   const equals = arg.indexOf("=");