Fix powermem-mcp help to exit without starting the server

[wayyoungboy]

Summary

Fixes #1120.

• Route powermem-mcp and python -m powermem.mcp through a lightweight CLI parser so --help exits before importing the MCP server.
• Keep normal MCP server startup behavior delegated to the existing server implementation, including the default transport, sse/stdio, custom ports, and invalid-port fallback.
• Update release binary entrypoint generation and smoke checks so packaged powermem-mcp --help uses the same lightweight path.

Validation

• python -m pytest tests/unit/test_mcp_cli.py tests/unit/test_release_binary_ci.py -q
• python -m pytest tests/unit/test_optional_dependency_imports.py tests/unit/server/test_main_mcp_optional.py -q
• python -m py_compile src/powermem/mcp/args.py src/powermem/mcp/cli.py src/powermem/mcp/server.py src/powermem/mcp/__main__.py tests/unit/test_mcp_cli.py scripts/smoke_binary_package.py tests/unit/test_release_binary_ci.py
• bash -n scripts/build_binary_package.sh
• powermem-mcp --help
• python -m powermem.mcp --help
• git diff --check
• python scripts/check_package_versions.py

The direct CLI E2E check returned 0, printed usage text, emitted no server-start log, and did not leave the default MCP port open.

Two independent code reviews found no blocking issues.