fix(testing): clear watch screen via Console, not deprecated os.system

[chfast]

🗒️ Description

mypy's deprecated error code (enabled in pyproject.toml) flags the os.system call in the --watch file watcher. Use the Rich Console.clear() the watcher already holds instead of shelling out to clear/cls; it is cross-platform and removes the last os use, so drop import os.

Why CI does not catch this: typeshed marks os.system @deprecated only for sys.version_info >= (3, 14), and [tool.mypy] pins no python_version, so mypy targets whatever interpreter runs it. The static CI job has no setup-python step (unlike the fill jobs, which pin 3.14) and runs mypy under the runner default, CPython 3.13, where the marker is inactive. Local dev on 3.14 sees the error; CI stays green.

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails, see also Code Standards and Verifying Changes:

  just static

• All: PR title have the form <type>(<area>):, where <type> and <area> come from an approrpriate C-<type>, respectively A-<area>, label. The title should match the a target squash commit message.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: Add the following docstring to manually enhanced tests from ./tests/ported_static/:

  @manually-enhanced: Do not overwrite. Post-state expectations corrected
  manually (see PR #2784).
  

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.30%. Comparing base (7603ff6) to head (fb06cfc).
⚠️ Report is 2 commits behind head on forks/amsterdam.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           forks/amsterdam    #3102      +/-   ##
===================================================
+ Coverage            93.24%   93.30%   +0.05%     
===================================================
  Files                  624      624              
  Lines                36986    36986              
  Branches              3383     3383              
===================================================
+ Hits                 34489    34511      +22     
+ Misses                1704     1693      -11     
+ Partials               793      782      -11     

Flag	Coverage Δ
unittests	93.30% <ø> (+0.05%)	⬆️

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[marioevz]

LGTM, thanks!