Allow diff pane to accept remote diff sources (GitHub PR URL, API, or unified diff)

Problem

The Desktop diff pane is currently limited to Claude's in-session file modifications. This makes it unusable as a review surface for external diffs — most notably GitHub pull requests.

For PR review workflows (e.g., a skill that walks through open PRs using the gh CLI and Jira MCP context), there's no native way to render the PR diff in a rich, GitHub-style viewer. The options today are:

• Dump gh pr diff output as text in the transcript (loses file tree, no syntax highlighting, no per-file navigation)
• Open the PR in the browser via gh pr view --web (leaves Claude Code entirely)
• Check out the PR branch locally (pollutes the working tree, which most review workflows explicitly avoid)

Proposed Solution

Extend the diff pane to accept remote diff sources. Possible input shapes:

• A GitHub PR URL or shorthand (e.g., owner/repo#1313)
• A GitHub API response body
• A generic unified-diff URL or file path
• An MCP-callable interface so skills can open a diff programmatically

Nice-to-haves

• File tree navigation (already present for session diffs)
• Configurable base ref (e.g., "diff against main" vs "diff against last-reviewed commit" for incremental re-reviews)
• Inline comments that write back as PR review comments via gh or an MCP tool
• Metadata sidebar: author, status, labels, CI checks, reviewers

Impact

This would turn Claude Code Desktop into a viable PR review surface and unlock rich skill-driven review workflows (triage, merge coordination, Jira integration, etc.) without forcing users to context-switch between Claude Code and a browser.