feat: add issue create and comment (task 0004)

Introduce the first mutations, along with the shared machinery the later
issue and PR mutation slices reuse.

- `issue create` with --title/--body/--body-file/--assignee/--label/
  --milestone/--fields, emitting `issue: { number, title, state, url }`
- `issue comment <n>`, echoing the created comment from the POST response
  with the body cleaned and truncated at 800 chars
- body-source resolution (--body vs --body-file), label and milestone
  name->ID lookup, repeatable flags, and the `joined`/`selectExtraFields`
  field extractors

Label lookup pages until exhausted, since a repo with more labels than one
page would otherwise fail to resolve a valid name. The end-to-end tier seeds
a mixed-case label and milestone and passes both in a different case, so the
case-insensitive lookup is verified against live Gitea rather than only
against fixtures.

test/harness.ts

@@ -1,4 +1,9 @@
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { expect } from "vitest";
 import { runCli } from "../src/cli.js";
+import type { FixtureServer } from "./fixture-server.js";
 
 export interface CliResult {
   stdout: string;
@@ -40,3 +45,40 @@ export function testModeEnv(apiUrl: string): Record<string, string> {
     GITEA_AXI_REPO: "testowner/testrepo",
   };
 }
+
+/**
+ * Throwaway files for the `--body-file` paths, cleaned up together. Call
+ * {@link TempFiles.write} to create one and {@link TempFiles.cleanup} from an
+ * `afterEach`.
+ */
+export interface TempFiles {
+  write: (name: string, content: string) => string;
+  cleanup: () => void;
+}
+
+export function tempFiles(): TempFiles {
+  const dirs: string[] = [];
+  return {
+    write: (name, content) => {
+      const dir = mkdtempSync(join(tmpdir(), "gitea-axi-test-"));
+      dirs.push(dir);
+      const path = join(dir, name);
+      writeFileSync(path, content, "utf8");
+      return path;
+    },
+    cleanup: () => {
+      for (const dir of dirs.splice(0)) {
+        rmSync(dir, { recursive: true, force: true });
+      }
+    },
+  };
+}
+
+/** The parsed body of the single POST the CLI sent to `path`; fails if it sent none. */
+export function postedBody(server: FixtureServer, path: string): Record<string, unknown> {
+  const post = server.requests.find(
+    (request) => request.method === "POST" && request.path === path,
+  );
+  expect(post, `expected a POST to ${path}`).toBeDefined();
+  return post!.body as Record<string, unknown>;
+}