test(ci): isolate semantic probe subprocess coverage

[Duang777]

Summary

• remove inherited pytest-cov state before launching the copied semantic-probe CLI
• keep synthetic checkout files out of shard coverage artifacts so aggregation can run on a separate runner

Root cause

The semantic-probe CLI tests execute copied loopx sources in a temporary checkout. Inheriting COV_CORE_* variables caused pytest-cov to record the synthetic loopx/a.py. The shard could report it while the temporary checkout existed, but the aggregate runner could not read that source path.

Verification

• 307 passed for tests/architecture/test_semantic_development_probe.py and tests/test_python_ci_workflow.py
• focused xdist + pytest-cov run produced no temporary source paths in the coverage database
• coverage reporting succeeded after removing the test temporary directory
• Ruff and git diff --check passed

This unblocks the baseline CI needed by #5367 and #5340.

[Duang777]

Exact-head CI attribution for 99c36b08345c6a279cef0a1422e352f55e41ec90:

• The target fix remains validated locally: all 17 semantic-probe tests passed under xdist + pytest-cov, the coverage database contained no temporary checkout paths, and coverage report --format=total succeeded after the temporary repository was removed.
• Remote pytest did not combine coverage. Its Require every upstream check step stopped after unrelated upstream failures, so this run neither reproduces nor disproves the former temporary-source aggregation error.
• typescript-core (1/3) failed on control_plane/work_items/task_lease_workspace.ts:26 /^[a-f0-9]{64}$/u, introduced by merged fix(coordination): allow isolated worktree edits with integration advisories #5364. The same check fails on current main run 36770551543.
• test-shard (2) and test-shard (3) failed on runtime fingerprint churn tests already fixed by fix(runtime): reject stale batched source fingerprints #5367.
• test-shard (3) also found a stale mainline assertion: merged feat(telemetry): qualify usage diagnostics and installation return cohorts #5366 raised _NOTICE_VERSION to 5, while test_source_cli_entrypoint.py still expects 4.
• All other functional jobs passed, including static checks, Windows, dashboard, both browser/bundle jobs, Stage2C, TypeScript 2/3 and 3/3, and Python shards 1 and 4.

This PR still changes only the semantic-probe subprocess environment. I left the branch unchanged and took no merge action.

[Duang777]

Closing this as a duplicate of #5375. Both PRs start from the same parent and apply the same semantic-probe subprocess coverage isolation to the same file; #5375 retains the explanatory boundary comment and is the canonical review target. No merge action was taken.