Fix #117: Remove Conan from Dev Container (unused?)

[leftibot]

Automated fix by @leftibot.

What changed

Fix #117: remove unused Conan install from dev container
Conan was installed in .devcontainer/Dockerfile but never used; the
project manages its dependencies via CPM (see Dependencies.cmake), so
the pip install conan step and the CONAN_SYSREQUIRES_* environment
variables were dead weight on every dev container build. Drop them, and
add a CTest regression test that scans the Dockerfile to prevent Conan
from being reintroduced.
Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

Files

 .devcontainer/Dockerfile                  | 14 --------------
 test/CMakeLists.txt                       |  6 ++++++
 test/check_no_conan_in_devcontainer.cmake | 19 +++++++++++++++++++
 3 files changed, 25 insertions(+), 14 deletions(-)

Closes #117

Triggered by @lefticus.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 20.50%. Comparing base (ec01c73) to head (c13d505).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   20.50%   20.50%           
=======================================
  Files           5        5           
  Lines         239      239           
  Branches      111      132   +21     
=======================================
  Hits           49       49           
+ Misses        186      154   -32     
- Partials        4       36   +32     

Flag	Coverage Δ
Linux	18.36% <ø> (ø)
Windows	21.02% <ø> (ø)
macOS	18.87% <ø> (ø)

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[lefticus]

@leftibot remove checks for the removal of Conan

[leftibot]

Addressed review feedback from @lefticus. New commits:

c13d505 Address review: remove devcontainer Conan-removal regression test