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.

test/help.test.ts

@@ -29,6 +29,23 @@ describe("--help", () => {
     expect(stdout).toContain("list");
   });
 
+  it("prints the pr group help and exits 0", async () => {
+    const { stdout, exitCode } = await runCliTest(["pr", "--help"]);
+
+    expect(exitCode).toBe(0);
+    expect(stdout).toContain("usage: gitea-axi pr");
+    expect(stdout).toContain("create");
+    expect(stdout).toContain("comment");
+  });
+
+  it("rejects an unknown pr subcommand with exit code 2", async () => {
+    const { stdout, exitCode } = await runCliTest(["pr", "frobnicate"]);
+
+    expect(exitCode).toBe(2);
+    expect(stdout).toContain("code: VALIDATION_ERROR");
+    expect(stdout).toContain("frobnicate");
+  });
+
   it("prints the version for --version", async () => {
     const { stdout, exitCode } = await runCliTest(["--version"]);