feat(tui): ProjectList fragment with lazy-loaded issue counts

[cdhagmann]

First Rooibos fragment in the TUI: lists the gems from the lockfile with their resolved hosts and an open-issue count for each.

Scope:

• GemContribute::TUI::ProjectList Rooibos fragment
• Lazy-load issue counts via Command.http
• Render gems with their host and (when known) issue count
• Pure-function Update test for key handlers and command-result messages

Sub-issue of #2 (TUI umbrella).

See: ADR-0013, docs/design.md TUI section, ROADMAP Phase 3