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/comment.ts

@@ -0,0 +1,40 @@
+import type { Comment } from "gitea-js";
+import { COMMENT_TRUNCATE_LIMIT, truncateBody } from "./body.js";
+import type { FlagSpec } from "./flags.js";
+import { relativeTime } from "./time.js";
+
+/**
+ * The one comment-posting shape `issue comment` and `pr comment` share. ADR 0008
+ * requires both to emit the same `comment` block with the same schema, so the
+ * block is built in exactly one place; what the two commands genuinely differ on
+ * — how a missing target is reported, and what to suggest next — stays with them.
+ */
+
+export const COMMENT_FLAGS: FlagSpec = {
+  "--body": { takesValue: true },
+  "--body-file": { takesValue: true },
+  "--full": { takesValue: false },
+};
+
+export interface CommentItemOptions {
+  /** The issue or pull request commented on — not the comment's own id. */
+  number: number;
+  /** Echo the body untruncated, as `--full` asks. */
+  full: boolean;
+  host: string;
+  now: Date;
+}
+
+/** The `comment: { number, author, created, body }` block, body truncated at 800 chars. */
+export function commentItem(
+  comment: Comment,
+  options: CommentItemOptions,
+): Record<string, unknown> {
+  const body = comment.body ?? "";
+  return {
+    number: options.number,
+    author: comment.user?.login ?? "",
+    created: relativeTime(comment.created_at, options.now),
+    body: options.full ? body : truncateBody(body, COMMENT_TRUNCATE_LIMIT, options.host),
+  };
+}