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/git.ts

@@ -43,6 +43,25 @@ export function parseRemoteUrl(url: string): RemoteRepo | null {
   return null;
 }
 
+/**
+ * The branch currently checked out, or null when git cannot name one — it is
+ * absent, this is not a repository, or HEAD is detached. Every null case is
+ * repaired the same way, by the caller naming the branch explicitly, so they
+ * are not distinguished here.
+ */
+export async function currentBranch(deps: CliDeps): Promise<string | null> {
+  const result = await runSubprocess("git", ["rev-parse", "--abbrev-ref", "HEAD"], {
+    cwd: deps.cwd,
+    env: deps.env,
+  });
+  if (result.enoent || result.code !== 0) {
+    return null;
+  }
+  const branch = result.stdout.trim();
+  // A detached HEAD abbreviates to the literal "HEAD", which is no branch.
+  return branch && branch !== "HEAD" ? branch : null;
+}
+
 export async function detectRemote(deps: CliDeps): Promise<RemoteRepo | null> {
   const result = await runSubprocess("git", ["remote", "get-url", "origin"], {
     cwd: deps.cwd,