feat: add pr merge and update-branch (task 0012)

Add `pr merge` with all six Gitea methods, the three common-method
shorthands, `--auto`, `--delete-branch`, `--subject`, `--body`/`--body-file`,
and `--merge-commit-id` (required with and only valid for
`--method manually-merged`). Conflicting method flags and the
manually-merged/commit-id pairing are rejected as VALIDATION_ERROR before
any API call. An already-merged PR short-circuits to a `pull_request` entity
block with `merged_by`/`merged_at`; merge-blocked 405/409 responses surface
as VALIDATION_ERROR with update-branch/checkout remediation.

Add `pr update-branch` merging the base into the head via the update
endpoint with `--style <merge|rebase>` (default merge).

test/pr-update-branch.test.ts

@@ -0,0 +1,66 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { startFixtureServer, type FixtureServer } from "./fixture-server.js";
+import { runCliTest, testModeEnv } from "./harness.js";
+
+const REPO_PATH = "/api/v1/repos/testowner/testrepo";
+const UPDATE_PATH = `${REPO_PATH}/pulls/9/update`;
+
+let server: FixtureServer;
+
+afterEach(async () => {
+  await server.close();
+});
+
+function updateRequest() {
+  return server.requests.find(
+    (request) => request.method === "POST" && request.path === UPDATE_PATH,
+  );
+}
+
+describe("pr update-branch", () => {
+  it("updates the head branch with the default merge style", async () => {
+    server = await startFixtureServer([{ method: "POST", path: UPDATE_PATH, body: {} }]);
+    const { stdout, exitCode } = await runCliTest(["pr", "update-branch", "9"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("updated:");
+    expect(stdout).toContain("number: 9");
+    expect(stdout).toContain("status: ok");
+    expect(updateRequest()?.query.style).toBe("merge");
+  });
+
+  it("passes --style rebase to the update endpoint", async () => {
+    server = await startFixtureServer([{ method: "POST", path: UPDATE_PATH, body: {} }]);
+    const { stdout, exitCode } = await runCliTest(["pr", "update-branch", "9", "--style", "rebase"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("updated:");
+    expect(updateRequest()?.query.style).toBe("rebase");
+  });
+
+  it("rejects an invalid --style value before any API call", async () => {
+    server = await startFixtureServer([]);
+    const { stdout, exitCode } = await runCliTest(["pr", "update-branch", "9", "--style", "cherry"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(2);
+    expect(stdout).toContain("code: VALIDATION_ERROR");
+    expect(server.requests).toHaveLength(0);
+  });
+
+  it("prints help with --help without calling the API", async () => {
+    server = await startFixtureServer([]);
+    const { stdout, exitCode } = await runCliTest(["pr", "update-branch", "--help"], {
+      env: testModeEnv(server.url),
+    });
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("usage: gitea-axi pr update-branch");
+    expect(server.requests).toHaveLength(0);
+  });
+});