Refactor dependencies for 'core-only' installation

[ryanleary]

Add Minimal Dependency Installation Support

What

Enables pip install garak to install only core dependencies for plugin discovery and config validation. Full dependencies for running scans require pip install garak[full].

Why

Users who only need to list probes or validate configs shouldn't need to install torch, transformers, and 40+ other heavy packages.

Changes

• pyproject.toml: Core deps (colorama, pyyaml, xdg-base-dirs, jinja2) as base; heavy deps moved to [full] extra
• Lazy imports: Deferred nltk, langdetect, requests, httpx, aiohttp to avoid import-time failures
• garak/discovery.py (new): Lightweight API for list_probes(), validate_config(), etc.
• CLI guards: Helpful error when attempting scans without full install

Testing

# 1. Minimal install - discovery works
python -m venv test_env && source test_env/bin/activate
pip install .
python -c "from garak.discovery import list_probes; print(len(list_probes()), 'probes')"

# 2. Scan attempt shows helpful error
python -m garak --target_type test --target_name Test
# Expected: ImportError with "pip install garak[full]" hint

# 3. Full install - scans work
pip install ".[full]"
python -m garak --target_type test --target_name Test --probes test.Blank

# 4. Existing tests pass
pip install ".[full,tests]" && pytest tests/

Breaking Change

pip install garak now installs minimal deps. Use pip install garak[full] for previous behavior.

[github-actions]

DCO Assistant Lite bot:
Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Developer Certificate of Origin before we can accept your contribution. You can sign the DCO by just posting a Pull Request Comment same as the below format.

---

I have read the DCO Document and I hereby sign the DCO

---

_{You can retrigger this bot by commenting recheck in this Pull Request}

[ryanleary]

@parkanzky @leondz fyi

[jmartin-tech]

There is already a different path for this #1475.