Redesign helix init: scaffold the helix layer, not the project

[amrit110]

Summary

• Removes solver.py and evaluate.py from the scaffold — helix should not prescribe codebase shape
• Adds pyproject.toml, .python-version, and uv.lock (via uv lock subprocess) for reproducible helices
• helix.yaml and program.md are now generic fill-in-the-blanks templates with clear TODOs
• scaffold() is non-destructive: existing files are never overwritten, making it safe to run against repos that already have a pyproject.toml or uv.lock
• run_uv_lock() extracted as a separate public function; CLI warns if uv is absent

Closes #14.

Test plan

• uv run python -m pytest — 199 passed
• helix init my-project creates helix.yaml, program.md, pyproject.toml, .python-version, uv.lock, experiments.tsv — no solver.py or evaluate.py
• Running helix init in a repo that already has pyproject.toml does not overwrite it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (6f0bd1a) to head (d78536f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   99.05%   99.06%   +0.01%     
==========================================
  Files          10       10              
  Lines         635      645      +10     
==========================================
+ Hits          629      639      +10     
  Misses          6        6              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.