fix: brand runtime as PytestPilot and repair Docker mount

[godkunzzz3]

What changed

• Replace runtime and TUI identity prompts/labels from FirstCoder with PytestPilot.
• Add a regression test for the default agent identity.
• Fix Docker bind mount syntax so the real Docker sandbox can start successfully.

Why

The CLI was still telling the model that it was FirstCoder, and Docker rejected the generated --mount argument with exit code 125.

Validation

• .venv/bin/python -m pytest tests -q
• Result: 904 passed, 2 skipped.