feat: auto-detect hardware when HELIX_HARDWARE is not set

[amrit110]

Closes #9

Summary

• Adds src/helix/hardware.py with detect_hardware() — a clean fallback chain:
  1. nvidia-smi → GPU name(s), deduplicated
  2. sysctl -n machdep.cpu.brand_string on macOS → Apple Silicon chip name
  3. platform.processor() → platform.machine() → "unknown"
• HelixRunner.run() calls it at session start and writes the result to HELIX_HARDWARE only if the env var isn't already set — explicit overrides always win
• 16 unit tests covering every path: GPU detection, Apple Silicon, all fallbacks, timeouts, empty output, and env var precedence

Test plan

• uv run pytest tests/test_hardware.py — 16 passed
• uv run pre-commit run --all-files — all checks passed
• Run helix run locally without HELIX_HARDWARE set — startup panel shows detected hardware instead of "unknown"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.07%. Comparing base (7a252a6) to head (a0a9355).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   99.03%   99.07%   +0.03%     
==========================================
  Files           9       10       +1     
  Lines         624      647      +23     
==========================================
+ Hits          618      641      +23     
  Misses          6        6              

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