Add prompts command with list, view, and delete subcommands

[parkerhendo]

Summary

• Add prompts command with list, view, and delete subcommands
• Use comfy_table for consistent table formatting
• Add pager support for long output
• Make base arguments (api-url, org-name) globally available

Test plan

• braintrust prompts list shows prompts in a table
• braintrust prompts view <name> opens prompt in web
• braintrust prompts delete <name> deletes after confirmation
• Table truncation handles emoji/unicode correctly

[github-actions]

Latest install commands for this PR (updated for commit 2b5741fdd828):

• Unix: curl -fsSL https://github.com/braintrustdata/bt/releases/download/canary-parker-prompts-cmd/bt-installer.sh | sh
• Windows: powershell -ExecutionPolicy Bypass -c "irm https://github.com/braintrustdata/bt/releases/download/canary-parker-prompts-cmd/bt-installer.ps1 | iex"