Add 'Generate PR Description' feature for automated PR title and description drafting

[creiney5]

This PR introduces a new feature to the Code Explainer VS Code extension that automatically generates a reviewable GitHub pull request title and description for the current branch. It leverages existing repository analysis and GitHub integration to draft PR descriptions tailored to different audiences and styles.

Changes Overview

• Added a new command codeExplainer.generatePrDescription to trigger PR description generation.
• Implemented PrDescriptionAnalysisService to analyze the current branch, generate draft PR titles and bodies, and interact with GitHub to create or update pull requests.
• Created a dedicated UI panel to display, edit, and apply generated PR descriptions.
• Extended the extension configuration with settings for default PR description style, guidelines, and templates.
• Integrated the new command into the extension activation lifecycle and command palette.
• Updated README and package.json to document and register the new feature.

Architectural Details

• The feature uses the PrDescriptionAnalysisService which combines repository scanning, prompt building for LLMs, and GitHub API interactions.
• The UI panel supports regenerating drafts with different styles or custom instructions and applying the final draft to GitHub.
• The system tracks PR state (no PR, existing empty, existing with description) to decide whether to create or update a PR.

Risks and Mitigations

• GitHub API interactions require authentication and network availability; errors are surfaced in the UI.
• Generated descriptions depend on LLM responses, which may vary; users can edit drafts before applying.
• The feature adds complexity to the extension; code is modularized and tested to isolate concerns.

Testing

• Manual testing was performed to verify generation, editing, and application of PR descriptions.
• Verified correct handling of existing PRs and branch states.
• UI interactions tested for responsiveness and error handling.

This description was generated using the new 'Generate PR Description' feature to demonstrate its capabilities.