test: add smoke tests for CLI subcommands

[Shreyas-Gowda26]

Closes #397

Added tests/test_cli_smoke.py covering CLI subcommand smoke tests:

• test_main_help_returns_zero: --help exits with code 0
• test_main_help_shows_subcommands: infer, landmarks, demo shown
• test_infer_help_returns_zero: infer --help exits with code 0
• test_infer_help_shows_flags: --procedure and --intensity flags shown
• test_landmarks_help_returns_zero: landmarks --help exits with code 0
• test_landmarks_help_shows_flags: --output flag shown
• test_demo_help_returns_zero: demo --help exits with code 0

No GPU or model weights required — help flags only.

[dreamlessx]

Welcome @Shreyas-Gowda26! Nice first contribution — clean smoke tests that cover all three CLI subcommands without needing GPU or model weights. This is exactly what #397 asked for.

CI is failing on one tiny issue — missing newline at end of file:

echo "" >> tests/test_cli_smoke.py
git add tests/test_cli_smoke.py
git commit -m "style: add trailing newline"
git push

Once that's fixed, I'll merge it right away!

[Shreyas-Gowda26]

Hey @dreamlessx Fixed! Added the trailing newline. Ready for review when you get a chance!

[dreamlessx]

Thanks for the fix @Shreyas-Gowda26! CI needs to re-run — waiting on workflow approval for the updated commit.

[dreamlessx]

Trailing newline is fixed, but CI is now failing on formatting — ruff format wants to reformat the file:

ruff format tests/test_cli_smoke.py
git add tests/test_cli_smoke.py
git commit -m "style: format test file"
git push

One more push and we're good!