Support helix init . and improve README

[amrit110]

Summary

• scaffold() now handles name="." correctly: resolves the actual directory name for template substitutions, so helix init . in an existing repo produces name: my-project in helix.yaml rather than name: .
• README "Writing your own helix" section rewritten to lead with the existing-repo workflow as the primary path, with from-scratch as a secondary option
• Em-dashes removed from README for consistent style

Test plan

• uv run python -m pytest — 201 passed
• helix init . in an existing directory creates helix files with the correct project name

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.07%. Comparing base (49fefb0) to head (3984132).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   99.06%   99.07%           
=======================================
  Files          10       10           
  Lines         645      646    +1     
=======================================
+ Hits          639      640    +1     
  Misses          6        6           

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