fix: honor updateRef when creating signed commits

[xxxxxxjun]

Summary

commit() ignored updateRef when a signature was given, leaving the signed
commit unreachable. The signed path now does what libgit2 already does for
unsigned commits.

Changes

• Resolve updateRef before writing the object, then point it at the new commit.
  Covers symbolic refs, unborn HEAD, and refs that don't exist yet.
• Reject a commit whose first parent isn't the current tip of updateRef. The
  check runs before commit_signed(), so a rejected commit leaves nothing in the odb.
• Record the same reflog entry as the unsigned path.

Notes

The first-parent check goes beyond the reported bug and turns a silent success
into an error. Without it, moving the ref on a mismatched first parent would let
a branch jump across history. Everything that pairs signature with updateRef
today (the existing test and the two usage/commit.md examples) already passes
the current tip.

Closes #218

[vercel]

@xxxxxxjun is attempting to deploy a commit to the Toss Team on Vercel.

A member of the Team first needs to authorize it.

[marshallku]

Thanks!