maintenance: CI migration, code hygiene, and test suite

[JakeBx]

maintenance: CI migration, code hygiene, and test suite

• Migrate from GitLab CI to GitHub Actions (.github/workflows/ci.yml)
  • pytest runs on every push/PR; Docker build+push gates on main/tag only
• Replace if/elif dispatch with registry dict in agents.py
  • _BUILDERS for agent types (simple/all_in_one/managing)
  • _TOOL_REGISTRY for tool sets (opencti/osint/attack/wikipedia/todays_date)
  • create_agent() collapses to a 3-line lookup
• Refactor tools/opencti_tools.py
  • Rename _filter_factory → _build_filter (returns dict, not callable)
  • Make client factory private: get_client() → _get_client()
  • Update all call-sites in opencti_write_tools.py
• Add pytest test suite (13 tests across agents and tools)
  • Fix mock path to tools.opencti_tools._get_client
  • Add test_build_filter_returns_well_formed_dict
  • Add test_builders_registry_contains_all_types
• Add pyproject.toml with [tool.pytest.ini_options] (pythonpath, testpaths)
  • Removes need for manual conftest.py sys.path hack
  • Add [tool.ruff] lint config
• Add .pre-commit-config.yaml with local pytest hook
• Delete .gitlab-ci.yml
• Update git remote origin to https://github.com/JakeBx/agentic-cti