fix(migrations): use configurable Jinja delimiters for default migration condition

[sisp]

I've fixed a bug related to rendering the default migration condition.

The default migration condition ({{ _stage == "after" }}) was built with hardcoded {{ and }} as variable start/end strings. When a template configured alternative delimiters via _envops such as

_envops:
  variable_start_string: "[["
  variable_end_string: "]]"

the default condition would contain the wrong delimiters and fail to render correctly.

The condition is now constructed using the delimiters resolved from the _envops settings in copier.yml, falling back to {{ and }} when not overridden.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.25%. Comparing base (f6ac946) to head (769a1a7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2584   +/-   ##
=======================================
  Coverage   97.24%   97.25%           
=======================================
  Files          59       59           
  Lines        6826     6850   +24     
=======================================
+ Hits         6638     6662   +24     
  Misses        188      188           

Flag	Coverage Δ
unittests	97.25% <100.00%> (+<0.01%)	⬆️

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