fix(add-wizard): preserve files and show recovery instructions on commit failure

[Copilot]

• Update commitChanges in git.go to capture git output for better error messages
• Update addWorkflowsWithPR in add_workflow_pr.go to not rollback files when commit fails, and print helpful instructions instead
• Fix unused errors import and trailing newline in error string that caused build failure and lint error
• Run fmt, build, lint, and tests to validate the changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>gh add-wizard: Failed to commit files in a repo</issue_title>
<issue_description>We had a cases where files failed to commit, we're not sure why. githubnext/agentics#204

We should give a better error message and exit, leaving the files uncomitted, exiting with a messsage saying that you should add and commit the files manually and either make a pull request or push them to the repository.

✓ Working directory is clean

✓ Selected engine: copilot
✓ Using existing COPILOT_GITHUB_TOKEN secret in repository

The following workflow files will be added:
• .github/workflows/repo-assist.md
• .github/workflows/repo-assist.lock.yml

✗ failed to add workflow: failed to commit files: failed to commit changes: exit status 1</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes gh add-wizard: Failed to commit files in a repo #18483

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[Copilot]

Pull request overview

Improves gh add-wizard failure handling when git commit fails, preserving generated workflow files and providing actionable recovery guidance instead of deleting files and returning a cryptic error.

Changes:

• Capture and surface git commit stderr/stdout in commitChanges errors via CombinedOutput().
• On commit failure during PR-based workflow add, stop rolling back files and print manual recovery steps.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
pkg/cli/git.go	Captures git commit output and includes it in returned errors for better diagnostics.
pkg/cli/add_workflow_pr.go	Replaces rollback-on-commit-failure with user guidance and preserves staged files for manual recovery.

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[dsyme]

@copilot fix this build error

[Copilot]

Fixed in 55e2360. The "errors" import was left over after switching from errors.New to fmt.Errorf (per the reviewer's suggestion). Also removed the trailing \n from the error format string to satisfy the staticcheck ST1005 rule.