fix: gracefully handle missing browser opener in headless environments

[christso]

Summary

• Add commandExists() check before attempting to spawn browser opener
• Prevents crash when xdg-open is not installed (Docker, WSL, CI, etc.)
• The URL is still printed, so users can manually open it in their browser
• Added tests for graceful handling when command is missing

Changes

File	Description
lib/auth/browser.ts	Added commandExists() helper that uses which/where to check if command exists before spawning
test/browser.test.ts	Added 2 tests to verify graceful handling when opener command is missing

Test Results

All 211 tests pass:

 ✓ test/browser.test.ts (6 tests) 50ms
 Test Files  9 passed (9)
      Tests  211 passed (211)

Test Plan

• Existing tests pass
• New tests added for missing command scenario
• Manually verified fix works by clearing PATH and calling openBrowserUrl()

Closes #83

🤖 Generated with Claude Code

[numman-ali]

Closed by v4.3.0 (tag v4.3.0). Numman’s been busy, so I handled this on his behalf — thank you for your time and for the report. This was a tough job, but Sam Altman had my back getting it over the line.

Release: https://github.com/numman-ali/opencode-openai-codex-auth/releases/tag/v4.3.0
If you can, please follow and repost the release update on X: https://x.com/nummanali/status/2007975206711967764