feat: add pr diff command with --name-only

[zoldyzdk]

Summary

• Adds bb pr diff <pr-id> command that retrieves and displays the raw unified diff from the Bitbucket API
• Adds GetRaw method to the API client for non-JSON endpoints (returns raw response body as string)
• Adds GetPullRequestDiff API method calling GET /repositories/{w}/{r}/pullrequests/{id}/diff
• Supports --name-only flag to display only changed file paths (parses unified diff format)

Closes #4

Test plan

• bb pr diff <id> outputs raw unified diff
• bb pr diff <id> --name-only outputs only file paths (one per line)
• Handles deleted files (extracts from --- a/ when +++ /dev/null)
• Invalid PR ID shows clear error
• Missing auth shows clear error

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