fix: make postProvision.sh venv cleanup non-fatal — closes #426

[sihbher]

The cleanup block at the end of postProvision.sh fails under set -euo pipefail when rm -rf cannot remove the Python 3.14 venv directory on macOS (file handles still held by recently completed python -m invocations).

• Add trap EXIT handler to ensure cleanup runs even on early script exits
• Replace rm -rf with python3 shutil.rmtree (handles locked files gracefully)
• Skip deactivate at script end (no shell state to restore)

Brings postProvision.sh to parity with the care already applied in postProvision.ps1, which has its cleanup commented out for the same reason.

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