fix(mcp): explain missing server extra

[JinNing6]

Summary

• route the brainctl-mcp console script through a narrow optional-dependency boundary
• print exact pip and pipx remediation when the [mcp] extra is missing, with exit code 1 and no traceback
• preserve unrelated import failures and normal server delegation
• make the README's core and MCP install paths explicit

Validation

• python -m pytest tests/test_mcp_entrypoint.py tests/test_doctor.py tests/test_mcp_allowed_tools.py -q — 26 passed
• built brainctl-2.8.0-py3-none-any.whl
• isolated base wheel install: exit 1, install hint present, no traceback
• isolated [mcp] wheel install: brainctl-mcp --help exits 0

Closes #159