feat: open in browser — `o` shortcut to open focused result's GitHub URL

[shouze]

Context

Part of the 🗂 TUI — Navigation & result management EPIC #66.

When browsing results in the TUI, users frequently want to open the file directly on GitHub. This feature adds an o shortcut that opens the focused result's GitHub URL in the default browser.

Acceptance criteria

• Pressing o on a focused result row opens its GitHub URL in the default browser
• macOS: uses open <url>, Linux: uses xdg-open <url>, Windows: uses start <url>
• If the focused row is a repository header or team section (no direct file URL), o is a no-op (or shows a brief status message)
• No blocking: the browser open command is spawned as a detached child process so the TUI remains responsive
• Keyboard shortcut is documented in docs/reference/keyboard-shortcuts.md

Implementation notes

• URL is available in CodeMatch.html_url (already present in the API response)
• Platform detection: process.platform === 'darwin' → open, 'linux' → xdg-open, 'win32' → start
• Use Bun.spawn (detached) or child_process.spawn — keep the spawn call isolated in a small helper so it can be tested via mock
• No new type changes needed

Branch

feat/tui-open-in-browser → merge into feat/tui-navigation-result-management

Validation

bun test
bun run lint
bun run format:check
bun run knip