git(core): include cwd and git output in pull_args error

Show the working directory and captured git output when `git pull`
fails via pull_args(). This makes debugging repository-specific
failures (missing upstream, auth issues, detached HEAD, etc.)
significantly easier, especially when pulling multiple repositories.

https://chatgpt.com/share/6969ff2c-ed2c-800f-b506-5834b6b81141

src/pkgmgr/core/git/commands/pull_args.py

@@ -29,7 +29,11 @@ def pull_args(
     try:
         run(["pull", *extra], cwd=cwd, preview=preview)
     except GitRunError as exc:
+        details = getattr(exc, "output", None) or getattr(exc, "stderr", None) or ""
         raise GitPullArgsError(
-            f"Failed to run `git pull` with args={extra!r}.",
+            (
+                f"Failed to run `git pull` with args={extra!r} "
+                f"in cwd={cwd!r}.\n{details}"
+            ).rstrip(),
             cwd=cwd,
         ) from exc