GitHub.com browser-like experience — explore repos, issues & PRs without leaving the app

### Feature summary

As a heavy user of the GitHub Copilot App, I'd love to be able to browse GitHub directly inside the app — without needing to jump back to a browser tab. The app already makes it so easy to work on code that the `github.com` browser experience starts to feel like a constant context switch.

### What problem are you trying to solve?

When working in the GitHub Copilot App, I find myself constantly switching to the browser to:
- Browse public or org repositories
- Check open issues and select one to work on
- Start a Copilot session directly from an issue
- See ongoing pull requests and jump into a review session

This back-and-forth breaks the flow. The app is so well crafted that I increasingly want to do **everything** from within it 🙂

### Proposed solution

Add a GitHub navigation surface inside the Copilot App that mirrors the core `github.com` browsing experience, including:

- **Repository explorer**: browse your own repos, your org's repos, and public/open-source repos (search, filter by org, stars, language, etc.)
- **Issues view**: list, filter, and read issues for any repo — with a direct CTA to **launch a Copilot session from an issue**
- **Pull Requests view**: list open PRs, see their diffs, statuses, and reviews — with a direct CTA to **open a PR review session**
- **Code navigation**: browse files and history for a repo, similar to github.com's file tree
- **Quick-open by URL**: paste a `github.com` URL and have the app open the corresponding view natively

### Workflow impact

Users can do the full GitHub workflow — discover → triage → code → review — entirely within the GitHub Copilot App, without ever opening a browser tab.

This is especially valuable for developer workflows already deeply integrated into the app (Copilot sessions from issues/PRs, worktrees, etc.). Having native repo/issue/PR navigation would make the app a **complete GitHub client**.

> Inspired by how great the current session-from-issue and PR review flows already feel in the app. This would be the natural next step

### Installation context

_No response_

### Additional context

_No response_

Start by reviewing the existing session-from-issue and PR review flows in the app to understand the current GitHub integration. Then map the proposed repository, issue, pull request, code-navigation, and URL-opening surfaces, and clarify which subset is in scope. Done would mean a defined, implemented browsing workflow that supports the selected views and launches the relevant sessions without leaving the app.

索引モデルが issue の本文から書いたものです。