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

[shouze]

Context

Part of the 🗂 TUI — Navigation & result management EPIC #66.
Closes #69.

Root cause / motivation

Users frequently want to open a result directly on GitHub without having to copy the URL. This adds a single keystroke to do it.

What changed

File	Change
src/tui.ts	New openInBrowser(url) helper: uses open (macOS), xdg-open (Linux) or start (Windows), spawned via Bun.spawn with all stdio null so the TUI stays fully responsive. o keypress: repo row → opens https://github.com/<org>/<repo>; extract row → opens CodeMatch.htmlUrl; section header → no-op.
src/render.ts	renderHelpOverlay: new o open in browser line. Status bar hint: o open.
src/render.test.ts	2 new tests: overlay contains open in browser; status bar contains o open.
docs/reference/keyboard-shortcuts.md	New row o in the Selection table.

How to test manually

bun github-code-search.ts query --org <your-org> <some-query>
# In the TUI:
#   Navigate to a repo row → press o → browser opens the repo page
#   Navigate to an extract row → press o → browser opens the file
#   Press ? → help overlay shows the o entry

Validation

bun test        ✅ 403 pass, 0 fail
bun run lint    ✅ 0 warnings, 0 errors
bun run format:check  ✅
bun run knip    ✅

[Copilot]

Pull request overview

This PR adds a single-keystroke o shortcut to the interactive TUI that opens the focused result's GitHub URL in the system default browser. It is part of the larger TUI Navigation & Result Management EPIC (#66).

Changes:

• New openInBrowser(url) helper in src/tui.ts that dispatches to the platform-appropriate open command, plus a new o keypress handler in the main input loop.
• Updated src/render.ts to surface the shortcut in both the help overlay and the status-bar hint line.
• Two new render tests (src/render.test.ts) verifying the overlay and status-bar text.
• New row in docs/reference/keyboard-shortcuts.md documenting the o key under the Selection table.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/tui.ts	Adds openInBrowser() helper and o keypress handler
src/render.ts	Adds o open in browser line to help overlay and status bar
src/render.test.ts	Tests that overlay and status bar contain the new shortcut text
docs/reference/keyboard-shortcuts.md	Documents the o shortcut in the Selection table

[shouze]

Both review points addressed in commit b38cc03:

1. Windows start as cmd.exe built-in — fixed by replacing ["start", url] with ["cmd", "/c", "start", "", url]. The empty string before the URL is required to prevent start from misinterpreting the URL as a window title.

2. JSDoc "detached" inaccuracy — replaced with "Fire-and-forget with all stdio set to null" which accurately describes the Bun.spawn call with { stdout: null, stderr: null, stdin: null }.

Also rebased onto the updated feat/tui-navigation-result-management (which now includes the merged PR #70 commits), keeping both the Z fold-all and o open entries in the status bar hint line and in the test suite (now 404 tests, all green).