chore: run test coverage improver twice daily (#3280)

* chore: run test coverage improver twice daily (8am/8pm UTC)

Change schedule from weekly to cron '0 8,20 * * *' to accelerate
test coverage improvements across the codebase.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address review feedback and prevent sudo awf hallucination

- Rename 'Weekly Test Coverage Improver' to 'Test Coverage Improver'
  (description, heading, and lock file name/run-name)
- Add explicit 'Do Not' rule: no sudo or awf CLI usage inside sandbox
  (addresses agent hallucinating 'sudo awf' in run #25974930350)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox