test: cover config loading behavior

[ynachiket]

Summary

• add dedicated unit coverage for config.py per Good First Issue: Write unit tests for config.py #65
• cover valid TOML loading, missing/invalid config fallback, and default copy isolation
• cover packaged default rule loading, AI rule inclusion, and shared placeholder substitution

Validation

• /tmp/pyspector-pdlc048-venv/bin/python -m pytest tests/unit/test_config.py -q
• /tmp/pyspector-pdlc048-venv/bin/python -m py_compile src/pyspector/config.py tests/unit/test_config.py
• /tmp/pyspector-pdlc048-venv/bin/python -m ruff check src/pyspector/config.py tests/unit/test_config.py
• git diff --check

Fixes #65

[ParzivalHack]

Hey @ynachiket, thanks for the PR and great idea adding deepcopy to prevent nesting/mutation bugs! Merging :D