feat: add validate-config subcommand (closes #9)

[qiaobochi040726-source]

Implements #9: add a validate-config subcommand that parses and validates stellargate.yaml (reusing Config.load error handling) and prints which tools are enabled, without running any scan.

Changes

• stellargate/cli.py: new validate-config subcommand (--config defaults to stellargate.yaml); config errors log to stderr and exit 2, success prints per-tool enabled/disabled summary and exits 0.
• tests/test_cli.py: 3 tests — valid config returns 0 with summary output, missing config returns 2 with error on stderr, and no scan is triggered.

Verification

• uv run --no-project --with pyyaml --with pytest --with hypothesis python -m pytest tests/ -q → 28 passed
• python -m stellargate.cli validate-config --config stellargate.example.yaml → exit 0
• python -m stellargate.cli validate-config --config /nonexistent.yaml → exit 2

Fixes #9