Add GitHub pull request update functionality with branch management

[csvenke]

Summary

Adds support for updating existing pull requests and automatic branch upstream management to the GitHub PR command.

Key Changes

New Features

• PR Update Detection: When creating a PR, the command now checks if a PR already exists and updates it instead of creating a duplicate
• Automatic Branch Push: Automatically pushes branches with upstream tracking (-u origin branch) when no upstream exists
• Enhanced PR Command Flow: Retrieves current branch and passes it through to PR creation logic

Implementation Details

• Added HasUpstream() and PushWithUpstream() methods to gh.Client interface
• Implemented git commands for upstream detection and branch pushing in RealClient
• Created CreatePullRequestWithBranch() function that handles branch management and PR creation/update logic
• Updated gh pr command to pass branch context to PR creation
• Modified test stub clients to support new interface methods

Behavior

• If a PR already exists: Updates title and body, returns "Pull request updated successfully"
• If no PR exists: Creates new PR using standard GitHub CLI
• Branch handling: Checks for upstream before pushing, skips push if upstream already exists
• Maintains backward compatibility with CreatePullRequest() wrapper function

Test Coverage

• Added comprehensive tests for CreatePullRequestWithBranch() covering:
  • Creating new PRs when none exist
  • Updating existing PRs
  • Branch push scenarios (with/without upstream)
  • Error handling for push and upstream check failures
  • Edge cases (empty branch, nil git client)