fix(cli): accept auto/mps devices in index build, with CI-safe test

[aafaq-rashid-comprinno]

Problem

pixelrag index build --device auto fails because argparse only allows cpu and cuda. PR #73 fixed the choices but its test fails in CI because pixelrag index build --help imports config.py → yaml which isn't installed in core-only CI.

Fix

1. Add auto, mps to --device choices
2. Test gracefully skips when the index stage isn't installed:

   if r.returncode != 0 and "not installed" in (r.stdout + r.stderr):
       pytest.skip("index stage not installed")

Supersedes #73 (same feature fix + working CI).

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