Agents Window should respect git.enableCommitSigning

When `git.enableCommitSigning` is enabled, commits created through the Source Control view are signed, but commits created by an agent in the Agents Window are not.

## Steps to reproduce

1. Configure a signing key in Git.
2. Enable `git.enableCommitSigning` in VS Code.
3. Create a commit through the Source Control view and confirm that it is signed.
4. Ask an agent in the Agents Window to create a commit.
5. Inspect the resulting commit signature.

## Actual behavior

The agent-created commit is unsigned. Agents invoke the Git CLI, where `user.signingKey` selects a key but does not enable signing unless `commit.gpgSign` is also configured or `git commit -S` is used. The VS Code `git.enableCommitSigning` setting is therefore not observed by the Agents Window.

This also affects agent-driven operations that rewrite commits, such as resolving conflicts through a rebase: the recreated commits remain unsigned.

## Expected behavior

Git operations initiated by the Agents Window should observe `git.enableCommitSigning`, so commits created or rewritten by an agent are signed consistently with commits created through the Source Control view.

Alternatively, the Agents Window should expose an equivalent signing policy and clearly explain how it relates to `git.enableCommitSigning` and Git\u2019s `commit.gpgSign` configuration.