chore: Delete dead lockdown code

[FrederikBolding]

Remove dead lockdown code from when we were manually running lockdown. This is no longer the case as the execution environments use LavaMoat.

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Note

Low Risk
Low risk: deletes a dead, unused helper and only affects code hygiene; remaining lockdown hardening continues to be handled via LavaMoat plus existing executeLockdownMore/executeLockdownEvents calls.

Overview
Removes the unused SES executeLockdown helper (src/common/lockdown/lockdown.ts), leaving lockdown initiation to LavaMoat and retaining only the extra hardening steps (executeLockdownMore and executeLockdownEvents) invoked by the execution environment entrypoints.

Updates coverage.json thresholds to reflect the small change in measured line/statement coverage.

^{Written by Cursor Bugbot for commit 31950ca. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (2887a53) to head (31950ca).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3841      +/-   ##
==========================================
+ Coverage   98.47%   98.59%   +0.11%     
==========================================
  Files         430      429       -1     
  Lines       12433    12417      -16     
  Branches     1929     1928       -1     
==========================================
- Hits        12243    12242       -1     
+ Misses        190      175      -15     

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