Add adversarial regression and performance suites

[codeforester]

Summary

Closes #68.

• Add deterministic Hypothesis property/fuzz coverage for command-protocol framing and argv/history redaction.
• Add spawn-based concurrency coverage for history appends, atomic metadata, shared log files, extension discovery caches, and run-bundle retention.
• Add lifecycle KeyboardInterrupt/Click Abort assertions plus a real POSIX SIGINT subprocess reproducer.
• Add a checked import/startup and per-invocation filesystem benchmark with documented p95 budgets.
• Document deterministic seeds and platform-specific signal/retention expectations.

Validation

• ./tests/validate.sh
• python scripts/validate_docs.py
• python scripts/benchmark_runtime.py --check
• python -m pytest -q
• python -m pytest --cov=base_cli --cov-report=term-missing --cov-fail-under=80 -q (83.45% total branch coverage)
• Ruff lint/format checks