feat: clarify empty picker states

[barrettruth]

Summary

• add compact scope counts to picker prompts so users can distinguish empty states from glitches
• render explicit placeholder rows for empty PR, issue, checks, CI, and release pickers instead of showing a blank list
• make all picker backends treat placeholder rows as non-selections so stacked list controls stay safe

Test plan

• nix develop .#ci --command stylua --check lua spec
• nix develop .#ci --command busted
• nix develop .#ci --command selene --display-style quiet lua/forge spec
• nix develop .#ci --command vimdoc-language-server check doc/