feat(tui): IssueDetail fragment with action keys (f / c / o)

[cdhagmann]

Issue detail view: body, labels, action keys for fixing, opening CONTRIBUTING, opening in browser.

Scope:

• GemContribute::TUI::IssueDetail Rooibos fragment
• Renders the issue body (markdown) and label set
• Action keys: f initiates fix flow, c opens ContributingViewer, o opens repo in browser
• Pure-function Update test for each key handler

Sub-issue of #2.

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