feat: add pr create and comment (task 0010)

`pr create` takes --title (required), --body/--body-file, --base, --head,
--assignee, --reviewer, repeatable name-resolved --label, and --milestone.
An omitted --head defaults to the current local branch; an omitted --base to
the repository's default branch. Before creating, an existing open PR for the
same base/head pair short-circuits to `pull_request: { number, url, already:
true }` rather than opening a duplicate; only an open PR does, since a closed
one's branches are free to be proposed again.

`pr comment <n>` posts through the shared issue-comment endpoint and returns
the created comment as `comment: { number, author, created, body }` (ADR 0008),
reporting a 404 as PR_NOT_FOUND since the caller asked about a pull request.

That comment block is now built in one place (src/comment.ts) for both issue
and pr comment, as ADR 0008 requires them to stay identical.

src/flags.ts

@@ -70,6 +70,44 @@ export function consumeFlagValue(
   return { value: next, lastIndex: index + 1 };
 }
 
+/** "issue" → "an issue"; "pull request" → "a pull request". */
+function withArticle(noun: string): string {
+  return /^[aeiou]/i.test(noun) ? `an ${noun}` : `a ${noun}`;
+}
+
+/**
+ * Parse the single positional number of a `<command> <number>` invocation.
+ * `noun` names what the number identifies ("issue", "pull request") and appears
+ * in the errors; the parsing itself is identical for both.
+ */
+export function parsePositionalNumber(
+  positionals: string[],
+  command: string,
+  noun: string,
+): number {
+  const helpSuggestion = [`Run \`gitea-axi ${command} --help\` to see available flags`];
+  if (positionals.length === 0) {
+    throw axiError(
+      `${command} requires ${withArticle(noun)} number`,
+      "VALIDATION_ERROR",
+      [`Run \`gitea-axi ${command} <number>\``],
+    );
+  }
+  if (positionals.length > 1) {
+    throw axiError(`Unexpected argument: ${positionals[1]}`, "VALIDATION_ERROR", helpSuggestion);
+  }
+  const raw = positionals[0]!;
+  const number = Number(raw);
+  if (!Number.isInteger(number) || number < 1) {
+    throw axiError(
+      `Invalid ${noun} number: ${raw} (expected a positive integer)`,
+      "VALIDATION_ERROR",
+      helpSuggestion,
+    );
+  }
+  return number;
+}
+
 export function parseFlags(
   args: string[],
   spec: FlagSpec,