Add setting to automatically delete branches after merge

[Copilot]

Implementation Complete: Delete Branch After Merge Setting

• Add new configuration setting githubPullRequests.deleteBranchAfterMerge in package.json
• Add localized string for the setting description in package.nls.json
• Add setting key constant in src/common/settingKeys.ts
• Modify merge logic in src/github/pullRequestOverview.ts to check setting and auto-delete branch
• Modify merge logic in src/github/activityBarViewProvider.ts to check setting and auto-delete branch
• Create autoDeleteBranchesAfterMerge function to silently delete branches without prompts
• Address code review feedback (extract helper, improve error handling, remove unrelated file)
• Refactor to reuse more of deleteBranch logic via shared performBranchDeletion helper
• Run security checks (CodeQL - no issues found)
• All linting and hygiene checks passed

Summary

This PR implements a new setting githubPullRequests.deleteBranchAfterMerge that automatically deletes branches after a pull request is successfully merged.

Key Features

• New Setting: githubPullRequests.deleteBranchAfterMerge (boolean, default: false)
• Silent Operation: Uses existing deletion preferences without showing prompts
• Respects User Preferences: Uses defaultDeletionMethod.selectLocalBranch and selectRemote settings
• Error Handling: Graceful error handling with logging - failures don't block merge
• Code Reuse: Extracted performBranchDeletion() helper shared by both interactive and automatic deletion

Technical Implementation

1. Added setting in package.json with localized description
2. Created performBranchDeletion() helper containing core deletion logic
3. autoDeleteBranchesAfterMerge() builds action list and calls shared helper
4. Interactive deleteBranch() now also uses shared helper (with user prompts for dirty trees)
5. Integrated auto-delete call in merge handlers (pullRequestOverview.ts and activityBarViewProvider.ts)

Security & Quality

• ✅ CodeQL security scan: No issues found
• ✅ ESLint: All checks passed
• ✅ Code hygiene: All checks passed
• ✅ Code review feedback addressed

Ready for final review and merge!

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature Request: Settings Option - Delete Branch after Merge</issue_title>
<issue_description>

Proposed Reasoning

My workplace has the policy of "delete branch after Peer Review and Merge", I can imagine others may have the same.
What I'm proposing is a settings option for Delete Branches after Merge

<label>Delete Branches after Merge</label>
<select>
    <option>Always Ask</option>
    <option>Do Not Automatically Delete(default)</option> 
    <option>Automatically Delete Local After Merge</option> 
    <option>Automatically Delete Local and Remote After Merge</option>
</select>

</issue_description>

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

• Fixes Feature Request: Settings Option - Delete Branch after Merge #1497

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[alexr00]

We should be able to re-use more of deleteBranch in pullRequestReviewCommon.ts isntead of fully having a new function.

[Copilot]

We should be able to re-use more of deleteBranch in pullRequestReviewCommon.ts isntead of fully having a new function.

Refactored to extract a shared performBranchDeletion() helper function that contains the core deletion logic. Both the interactive deleteBranch() and automatic autoDeleteBranchesAfterMerge() now use this shared helper, eliminating code duplication. The interactive version retains special handling for dirty working trees with user prompts. (4cb17f1)