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/e2e/provision.ts

@@ -219,6 +219,37 @@ export async function provisionInstance(baseUrl: string): Promise<E2EInstance> {
   };
 }
 
+/**
+ * Create `branch` off the default branch, carrying one new file, so that a pull
+ * request opened from it has a real diff to propose.
+ */
+export async function seedBranch(instance: E2EInstance, branch: string): Promise<void> {
+  await apiRequest(
+    instance.baseUrl,
+    "POST",
+    `/repos/${instance.owner}/${instance.repo}/contents/${branch}.txt`,
+    instance.token,
+    {
+      content: Buffer.from(`Seeded on ${branch}.\n`).toString("base64"),
+      message: `Seed ${branch}`,
+      new_branch: branch,
+    },
+  );
+}
+
+/** Fetch the repository's open pull requests as Gitea returns them. */
+export async function fetchOpenPulls(
+  instance: E2EInstance,
+): Promise<Record<string, unknown>[]> {
+  const res = await apiRequest(
+    instance.baseUrl,
+    "GET",
+    `/repos/${instance.owner}/${instance.repo}/pulls?state=open`,
+    instance.token,
+  );
+  return (await res.json()) as Record<string, unknown>[];
+}
+
 /** Fetch a single issue as Gitea returns it, for verifying what a mutation wrote. */
 export async function fetchIssue(
   instance: E2EInstance,