Add --no-gpg-sign to codeflow git commits

[Copilot]

darc vmr commands (e.g. resolve-conflict) make temporary local commits during codeflow. Users with GPG signing enabled globally get interrupted by signing-password prompts mid-operation.

All codeflow commits route through a small set of central git commit invocations, each now passes --no-gpg-sign:

• LocalGitClient.CommitAsync — primary commit path used by all Vmr flowers and conflict resolvers (VmrManagerBase, WorkBranch, VmrBackFlower, CodeFlowConflictResolver, VmrCodeflower).
• LocalGitClient.CommitAmendAsync — amend commits.
• RemoteRepoBase — the remaining direct git commit in the PR-creation path.

IEnumerable<string> args = new[] { "commit", "--no-gpg-sign", "-m", message };

Merge/rebase steps in WorkBranch use --no-commit/--squash and finalize via CommitAsync, so they're covered by the central change.

#6266

[Copilot]

Pull request overview

This PR updates DARC’s VMR/codeflow-related git commit invocations to explicitly disable GPG signing (--no-gpg-sign), preventing interactive signing/passphrase prompts during temporary/local commits that can interrupt darc vmr workflows.

Changes:

• Add --no-gpg-sign to LocalGitClient.CommitAsync argument construction.
• Add --no-gpg-sign to LocalGitClient.CommitAmendAsync.
• Add --no-gpg-sign to the remaining direct git commit invocation in RemoteRepoBase.CommitFilesAsync.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.DotNet.Darc/DarcLib/RemoteRepoBase.cs	Disables GPG signing for commit created during remote repo file commit/push flow.
src/Microsoft.DotNet.Darc/DarcLib/LocalGitClient.cs	Disables GPG signing for standard and amend commit paths used by VMR/codeflow operations.